Added undo/redo and made entity translation undoable.

observable/src/commonMain/kotlin/world/phantasmal/observable/value/Val.kt

@@ -18,15 +18,36 @@ interface Val<out T> : Observable<T> {
      */
     fun observe(callNow: Boolean = false, observer: Observer<T>): Disposable
 
+    /**
+     * Map a transformation function over this val.
+     *
+     * @param transform called whenever this val changes
+     */
     fun <R> map(transform: (T) -> R): Val<R> =
         MappedVal(listOf(this)) { transform(value) }
 
+    /**
+     * Map a transformation function over this val and another val.
+     *
+     * @param transform called whenever this val or [v2] changes
+     */
     fun <T2, R> map(v2: Val<T2>, transform: (T, T2) -> R): Val<R> =
         MappedVal(listOf(this, v2)) { transform(value, v2.value) }
 
+    /**
+     * Map a transformation function over this val and two other vals.
+     *
+     * @param transform called whenever this val, [v2] or [v3] changes
+     */
     fun <T2, T3, R> map(v2: Val<T2>, v3: Val<T3>, transform: (T, T2, T3) -> R): Val<R> =
         MappedVal(listOf(this, v2, v3)) { transform(value, v2.value, v3.value) }
 
+    /**
+     * Map a transformation function that returns a val over this val. The resulting val will change
+     * when this val changes and when the val returned by [transform] changes.
+     *
+     * @param transform called whenever this val changes
+     */
     fun <R> flatMap(transform: (T) -> Val<R>): Val<R> =
         FlatMappedVal(listOf(this)) { transform(value) }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/value/ValExtensions.kt

@@ -1,5 +1,35 @@
 package world.phantasmal.observable.value
 
+infix fun Val<Any?>.eq(value: Any?): Val<Boolean> =
+    map { it == value }
+
+infix fun Val<Any?>.eq(value: Val<Any?>): Val<Boolean> =
+    map(value) { a, b -> a == b }
+
+infix fun Val<Any?>.ne(value: Any?): Val<Boolean> =
+    map { it != value }
+
+infix fun Val<Any?>.ne(value: Val<Any?>): Val<Boolean> =
+    map(value) { a, b -> a != b }
+
+fun Val<Any?>.isNull(): Val<Boolean> =
+    map { it == null }
+
+fun Val<Any?>.isNotNull(): Val<Boolean> =
+    map { it != null }
+
+infix fun <T : Comparable<T>> Val<T>.gt(value: T): Val<Boolean> =
+    map { it > value }
+
+infix fun <T : Comparable<T>> Val<T>.gt(value: Val<T>): Val<Boolean> =
+    map(value) { a, b -> a > b }
+
+infix fun <T : Comparable<T>> Val<T>.lt(value: T): Val<Boolean> =
+    map { it < value }
+
+infix fun <T : Comparable<T>> Val<T>.lt(value: Val<T>): Val<Boolean> =
+    map(value) { a, b -> a < b }
+
 infix fun Val<Boolean>.and(other: Val<Boolean>): Val<Boolean> =
     map(other) { a, b -> a && b }
 

observable/src/commonMain/kotlin/world/phantasmal/observable/value/list/AbstractListVal.kt

@@ -6,9 +6,6 @@ import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Observable
 import world.phantasmal.observable.Observer
 import world.phantasmal.observable.value.AbstractVal
-import world.phantasmal.observable.value.MutableVal
-import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.mutableVal
 
 abstract class AbstractListVal<E>(
     protected val elements: MutableList<E>,
@@ -25,6 +22,9 @@ abstract class AbstractListVal<E>(
      */
     protected val listObservers = mutableListOf<ListValObserver<E>>()
 
+    override fun get(index: Int): E =
+        elements[index]
+
     override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable {
         if (elementObservers.isEmpty() && extractObservables != null) {
             replaceElementObservers(0, elementObservers.size, elements)

observable/src/commonMain/kotlin/world/phantasmal/observable/value/list/ListVal.kt

@@ -6,6 +6,8 @@ import world.phantasmal.observable.value.Val
 interface ListVal<E> : Val<List<E>> {
     val sizeVal: Val<Int>
 
+    operator fun get(index: Int): E
+
     fun observeList(callNow: Boolean = false, observer: ListValObserver<E>): Disposable
 
     fun sumBy(selector: (E) -> Int): Val<Int> =

observable/src/commonMain/kotlin/world/phantasmal/observable/value/list/MutableListVal.kt

@@ -3,7 +3,7 @@ package world.phantasmal.observable.value.list
 import world.phantasmal.observable.value.MutableVal
 
 interface MutableListVal<E> : ListVal<E>, MutableVal<List<E>> {
-    fun set(index: Int, element: E): E
+    operator fun set(index: Int, element: E): E
 
     fun add(element: E)
 
@@ -15,5 +15,7 @@ interface MutableListVal<E> : ListVal<E>, MutableVal<List<E>> {
 
     fun replaceAll(elements: Sequence<E>)
 
+    fun splice(from: Int, removeCount: Int, newElement: E)
+
     fun clear()
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/value/list/SimpleListVal.kt

@@ -25,7 +25,10 @@ class SimpleListVal<E>(
 
     override val sizeVal: Val<Int> = _sizeVal
 
-    override fun set(index: Int, element: E): E {
+    override operator fun get(index: Int): E =
+        elements[index]
+
+    override operator fun set(index: Int, element: E): E {
         val removed = elements.set(index, element)
         finalizeUpdate(ListValChangeEvent.Change(index, listOf(removed), listOf(element)))
         return removed
@@ -62,6 +65,13 @@ class SimpleListVal<E>(
         finalizeUpdate(ListValChangeEvent.Change(0, removed, this.elements))
     }
 
+    override fun splice(from: Int, removeCount: Int, newElement: E) {
+        val removed = ArrayList(elements.subList(from, from + removeCount))
+        repeat(removeCount) { elements.removeAt(from) }
+        elements.add(from, newElement)
+        finalizeUpdate(ListValChangeEvent.Change(from, removed, listOf(newElement)))
+    }
+
     override fun clear() {
         val removed = ArrayList(elements)
         elements.clear()

observable/src/commonMain/kotlin/world/phantasmal/observable/value/list/StaticListVal.kt

@@ -7,11 +7,14 @@ import world.phantasmal.observable.Observer
 import world.phantasmal.observable.value.Val
 import world.phantasmal.observable.value.value
 
-class StaticListVal<E>(elements: List<E>) : ListVal<E> {
+class StaticListVal<E>(private val elements: List<E>) : ListVal<E> {
     override val sizeVal: Val<Int> = value(elements.size)
 
     override val value: List<E> = elements
 
+    override fun get(index: Int): E =
+        elements[index]
+
     override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable {
         if (callNow) {
             observer(ChangeEvent(value))

observable/src/commonTest/kotlin/world/phantasmal/observable/ObservableTests.kt

@@ -4,14 +4,20 @@ import world.phantasmal.observable.test.ObservableTestSuite
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
-typealias ObservableAndEmit = Pair<Observable<*>, () -> Unit>
+open class ObservableAndEmit<T, out O : Observable<T>>(
+    val observable: O,
+    val emit: () -> Unit,
+) {
+    operator fun component1() = observable
+    operator fun component2() = emit
+}
 
 /**
  * Test suite for all [Observable] implementations. There is a subclass of this suite for every
  * [Observable] implementation.
  */
 abstract class ObservableTests : ObservableTestSuite() {
-    protected abstract fun create(): ObservableAndEmit
+    protected abstract fun create(): ObservableAndEmit<*, Observable<*>>
 
     @Test
     fun observable_calls_observers_when_events_are_emitted() = test {

observable/src/commonTest/kotlin/world/phantasmal/observable/SimpleEmitterTests.kt

@@ -1,7 +1,7 @@
 package world.phantasmal.observable
 
 class SimpleEmitterTests : ObservableTests() {
-    override fun create(): ObservableAndEmit {
+    override fun create(): ObservableAndEmit<*, SimpleEmitter<*>> {
         val observable = SimpleEmitter<Any>()
         return ObservableAndEmit(observable) { observable.emit(ChangeEvent(Any())) }
     }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/DelegatingValTests.kt

@@ -1,15 +1,16 @@
 package world.phantasmal.observable.value
 
+import world.phantasmal.observable.ObservableAndEmit
+
 class DelegatingValTests : RegularValTests() {
-    override fun create(): ValAndEmit<*> {
+    override fun create(): ObservableAndEmit<*, DelegatingVal<*>> {
         var v = 0
         val value = DelegatingVal({ v }, { v = it })
-        return ValAndEmit(value) { value.value += 2 }
+        return ObservableAndEmit(value) { value.value += 2 }
     }
 
-    override fun createBoolean(bool: Boolean): ValAndEmit<Boolean> {
-        var v = bool
-        val value = DelegatingVal({ v }, { v = it })
-        return ValAndEmit(value) { value.value = !value.value }
+    override fun <T> createWithValue(value: T): DelegatingVal<T> {
+        var v = value
+        return DelegatingVal({ v }, { v = it })
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/FlatMappedValDependentValEmitsTests.kt

@@ -1,5 +1,6 @@
 package world.phantasmal.observable.value
 
+import world.phantasmal.observable.ObservableAndEmit
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNull
@@ -33,15 +34,14 @@ class FlatMappedValDependentValEmitsTests : RegularValTests() {
         assertEquals(7, observedValue)
     }
 
-    override fun create(): ValAndEmit<*> {
+    override fun create(): ObservableAndEmit<*, FlatMappedVal<*>> {
         val v = SimpleVal(SimpleVal(5))
         val value = FlatMappedVal(listOf(v)) { v.value }
-        return ValAndEmit(value) { v.value = SimpleVal(v.value.value + 5) }
+        return ObservableAndEmit(value) { v.value = SimpleVal(v.value.value + 5) }
     }
 
-    override fun createBoolean(bool: Boolean): ValAndEmit<Boolean> {
-        val v = SimpleVal(SimpleVal(bool))
-        val value = FlatMappedVal(listOf(v)) { v.value }
-        return ValAndEmit(value) { v.value = SimpleVal(!v.value.value) }
+    override fun <T> createWithValue(value: T): FlatMappedVal<T> {
+        val v = SimpleVal(SimpleVal(value))
+        return FlatMappedVal(listOf(v)) { v.value }
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/FlatMappedValNestedValEmitsTests.kt

@@ -1,18 +1,19 @@
 package world.phantasmal.observable.value
 
+import world.phantasmal.observable.ObservableAndEmit
+
 /**
  * In these tests the dependency of the [FlatMappedVal]'s direct dependency changes.
  */
 class FlatMappedValNestedValEmitsTests : RegularValTests() {
-    override fun create(): ValAndEmit<*> {
+    override fun create(): ObservableAndEmit<*, FlatMappedVal<*>> {
         val v = SimpleVal(SimpleVal(5))
         val value = FlatMappedVal(listOf(v)) { v.value }
-        return ValAndEmit(value) { v.value.value += 5 }
+        return ObservableAndEmit(value) { v.value.value += 5 }
     }
 
-    override fun createBoolean(bool: Boolean): ValAndEmit<Boolean> {
-        val v = SimpleVal(SimpleVal(bool))
-        val value = FlatMappedVal(listOf(v)) { v.value }
-        return ValAndEmit(value) { v.value.value = !v.value.value }
+    override fun <T> createWithValue(value: T): FlatMappedVal<T> {
+        val v = SimpleVal(SimpleVal(value))
+        return FlatMappedVal(listOf(v)) { v.value }
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/MappedValTests.kt

@@ -1,15 +1,16 @@
 package world.phantasmal.observable.value
 
+import world.phantasmal.observable.ObservableAndEmit
+
 class MappedValTests : RegularValTests() {
-    override fun create(): ValAndEmit<*> {
+    override fun create(): ObservableAndEmit<*, MappedVal<*>> {
         val v = SimpleVal(0)
         val value = MappedVal(listOf(v)) { 2 * v.value }
-        return ValAndEmit(value) { v.value += 2 }
+        return ObservableAndEmit(value) { v.value += 2 }
     }
 
-    override fun createBoolean(bool: Boolean): ValAndEmit<Boolean> {
-        val v = SimpleVal(bool)
-        val value = MappedVal(listOf(v)) { v.value }
-        return ValAndEmit(value) { v.value = !v.value }
+    override fun <T> createWithValue(value: T): MappedVal<T> {
+        val v = SimpleVal(value)
+        return MappedVal(listOf(v)) { v.value }
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/RegularValTests.kt

@@ -10,12 +10,73 @@ import kotlin.test.assertTrue
  * for every non-ListVal [Val] implementation.
  */
 abstract class RegularValTests : ValTests() {
-    protected abstract fun createBoolean(bool: Boolean): ValAndEmit<Boolean>
+    protected abstract fun <T> createWithValue(value: T): Val<T>
+
+    @Test
+    fun val_any_extensions() = test {
+        listOf(Any(), null).forEach { any ->
+            val value = createWithValue(any)
+
+            // Test the test setup first.
+            assertEquals(any, value.value)
+
+            // Test `isNull`.
+            assertEquals(any == null, value.isNull().value)
+
+            // Test `isNotNull`.
+            assertEquals(any != null, value.isNotNull().value)
+        }
+        listOf(10 to 10, 5 to 99, "a" to "a", "x" to "y").forEach { (a, b) ->
+            val aVal = createWithValue(a)
+            val bVal = createWithValue(b)
+
+            // Test the test setup first.
+            assertEquals(a, aVal.value)
+            assertEquals(b, bVal.value)
+
+            // Test `eq`.
+            assertEquals(a == b, (aVal eq b).value)
+            assertEquals(a == b, (aVal eq bVal).value)
+
+            // Test `ne`.
+            assertEquals(a != b, (aVal ne b).value)
+            assertEquals(a != b, (aVal ne bVal).value)
+        }
+    }
+
+    @Test
+    fun val_comparable_extensions() = test {
+        listOf(
+            10 to 10,
+            7.0 to 5.0,
+            (5000).toShort() to (7000).toShort()
+        ).forEach { (a, b) ->
+            @Suppress("UNCHECKED_CAST")
+            a as Comparable<Any>
+            @Suppress("UNCHECKED_CAST")
+            b as Comparable<Any>
+
+            val aVal = createWithValue(a)
+            val bVal = createWithValue(b)
+
+            // Test the test setup first.
+            assertEquals(a, aVal.value)
+            assertEquals(b, bVal.value)
+
+            // Test `gt`.
+            assertEquals(a > b, (aVal gt b).value)
+            assertEquals(a > b, (aVal gt bVal).value)
+
+            // Test `lt`.
+            assertEquals(a < b, (aVal lt b).value)
+            assertEquals(a < b, (aVal lt bVal).value)
+        }
+    }
 
     @Test
     fun val_boolean_extensions() = test {
         listOf(true, false).forEach { bool ->
-            val (value) = createBoolean(bool)
+            val value = createWithValue(bool)
 
             // Test the test setup first.
             assertEquals(bool, value.value)

observable/src/commonTest/kotlin/world/phantasmal/observable/value/SimpleValTests.kt

@@ -1,13 +1,13 @@
 package world.phantasmal.observable.value
 
+import world.phantasmal.observable.ObservableAndEmit
+
 class SimpleValTests : RegularValTests() {
-    override fun create(): ValAndEmit<*> {
+    override fun create(): ObservableAndEmit<*, SimpleVal<*>> {
         val value = SimpleVal(1)
-        return ValAndEmit(value) { value.value += 2 }
+        return ObservableAndEmit(value) { value.value += 2 }
     }
 
-    override fun createBoolean(bool: Boolean): ValAndEmit<Boolean> {
-        val value = SimpleVal(bool)
-        return ValAndEmit(value) { value.value = !value.value }
-    }
+    override fun <T> createWithValue(value: T): SimpleVal<T> =
+        SimpleVal(value)
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/ValTests.kt

@@ -1,18 +1,18 @@
 package world.phantasmal.observable.value
 
 import world.phantasmal.core.disposable.use
+import world.phantasmal.observable.Observable
+import world.phantasmal.observable.ObservableAndEmit
 import world.phantasmal.observable.ObservableTests
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
-typealias ValAndEmit<T> = Pair<Val<T>, () -> Unit>
-
 /**
  * Test suite for all [Val] implementations. There is a subclass of this suite for every [Val]
  * implementation.
  */
 abstract class ValTests : ObservableTests() {
-    abstract override fun create(): ValAndEmit<*>
+     abstract override fun create(): ObservableAndEmit<*, Val<*>>
 
     /**
      * When [Val.observe] is called with callNow = true, it should call the observer immediately.

observable/src/commonTest/kotlin/world/phantasmal/observable/value/list/DependentListValTests.kt

@@ -1,7 +1,7 @@
 package world.phantasmal.observable.value.list
 
 class DependentListValTests : ListValTests() {
-    override fun create(): ListValAndAdd {
+    override fun create(): ListValAndAdd<*, DependentListVal<*>> {
         val l = SimpleListVal<Int>(mutableListOf())
         val list = DependentListVal(listOf(l)) { l.value.map { 2 * it } }
         return ListValAndAdd(list) { l.add(4) }

observable/src/commonTest/kotlin/world/phantasmal/observable/value/list/ListValTests.kt

@@ -1,17 +1,21 @@
 package world.phantasmal.observable.value.list
 
+import world.phantasmal.observable.ObservableAndEmit
 import world.phantasmal.observable.value.ValTests
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
-typealias ListValAndAdd = Pair<ListVal<*>, () -> Unit>
+class ListValAndAdd<T, out O : ListVal<T>>(
+    observable: O,
+    add: () -> Unit,
+) : ObservableAndEmit<List<T>, O>(observable, add)
 
 /**
  * Test suite for all [ListVal] implementations. There is a subclass of this suite for every
  * [ListVal] implementation.
  */
 abstract class ListValTests : ValTests() {
-    abstract override fun create(): ListValAndAdd
+    abstract override fun create(): ListValAndAdd<*, ListVal<*>>
 
     @Test
     fun listVal_updates_sizeVal_correctly() = test {

observable/src/commonTest/kotlin/world/phantasmal/observable/value/list/SimpleListValTests.kt

@@ -1,7 +1,7 @@
 package world.phantasmal.observable.value.list
 
 class SimpleListValTests : ListValTests() {
-    override fun create(): ListValAndAdd {
+    override fun create(): ListValAndAdd<*, SimpleListVal<*>> {
         val value = SimpleListVal(mutableListOf<Int>())
         return ListValAndAdd(value) { value.add(7) }
     }

web/src/main/kotlin/world/phantasmal/web/application/Application.kt

@@ -50,7 +50,7 @@ class Application(
         // The various tools Phantasmal World consists of.
         val tools: List<PwTool> = listOf(
             Viewer(createEngine),
-            QuestEditor(assetLoader, createEngine),
+            QuestEditor(assetLoader, uiStore, createEngine),
             HuntOptimizer(assetLoader, uiStore),
         )
 

web/src/main/kotlin/world/phantasmal/web/application/widgets/MainContentWidget.kt

@@ -2,9 +2,7 @@ package world.phantasmal.web.application.widgets
 
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
-import world.phantasmal.observable.value.not
 import world.phantasmal.web.application.controllers.MainContentController
-import world.phantasmal.web.core.PwTool
 import world.phantasmal.web.core.PwToolType
 import world.phantasmal.webui.dom.div
 import world.phantasmal.webui.widgets.LazyLoader
@@ -21,7 +19,7 @@ class MainContentWidget(
 
             ctrl.tools.forEach { (tool, active) ->
                 toolViews[tool]?.let { createWidget ->
-                    addChild(LazyLoader(scope, hidden = !active, createWidget = createWidget))
+                    addChild(LazyLoader(scope, visible = active, createWidget = createWidget))
                 }
             }
         }

web/src/main/kotlin/world/phantasmal/web/application/widgets/NavigationWidget.kt

@@ -2,7 +2,8 @@ package world.phantasmal.web.application.widgets
 
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
-import world.phantasmal.observable.value.trueVal
+import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.value
 import world.phantasmal.web.application.controllers.NavigationController
 import world.phantasmal.web.core.dom.externalLink
 import world.phantasmal.webui.dom.Icon
@@ -31,11 +32,11 @@ class NavigationWidget(
 
                 val serverSelect = Select(
                     scope,
-                    disabled = trueVal(),
+                    enabled = falseVal(),
                     label = "Server:",
                     items = listOf("Ephinea"),
                     selected = "Ephinea",
-                    tooltip = "Only Ephinea is supported at the moment",
+                    tooltip = value("Only Ephinea is supported at the moment"),
                 )
                 addWidget(serverSelect.label!!)
                 addChild(serverSelect)

web/src/main/kotlin/world/phantasmal/web/core/actions/Action.kt

@@ -0,0 +1,7 @@
+package world.phantasmal.web.core.actions
+
+interface Action {
+    val description: String
+    fun execute()
+    fun undo()
+}

web/src/main/kotlin/world/phantasmal/web/core/controllers/PathAwareTabController.kt

@@ -30,10 +30,10 @@ open class PathAwareTabController<T : PathAwareTab>(
         super.setActiveTab(tab)
     }
 
-    override fun hiddenChanged(hidden: Boolean) {
-        super.hiddenChanged(hidden)
+    override fun visibleChanged(visible: Boolean) {
+        super.visibleChanged(visible)
 
-        if (!hidden && uiStore.currentTool.value == tool) {
+        if (visible && uiStore.currentTool.value == tool) {
             activeTab.value?.let {
                 uiStore.setPathPrefix(it.path, replace = true)
             }

web/src/main/kotlin/world/phantasmal/web/core/stores/UiStore.kt

@@ -5,6 +5,7 @@ import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.events.KeyboardEvent
 import world.phantasmal.observable.value.MutableVal
 import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.eq
 import world.phantasmal.observable.value.mutableVal
 import world.phantasmal.web.core.PwToolType
 import world.phantasmal.web.core.models.Server
@@ -76,7 +77,7 @@ class UiStore(
 
         toolToActive = tools
             .map { tool ->
-                tool to currentTool.map { it == tool }
+                tool to (currentTool eq tool)
             }
             .toMap()
 

web/src/main/kotlin/world/phantasmal/web/core/undo/Undo.kt

@@ -0,0 +1,27 @@
+package world.phantasmal.web.core.undo
+
+import world.phantasmal.observable.value.Val
+import world.phantasmal.web.core.actions.Action
+
+interface Undo {
+    val canUndo: Val<Boolean>
+    val canRedo: Val<Boolean>
+
+    /**
+     * The first action that will be undone when calling undo().
+     */
+    val firstUndo: Val<Action?>
+
+    /**
+     * The first action that will be redone when calling redo().
+     */
+    val firstRedo: Val<Action?>
+
+    /**
+     * Ensures this undo is the current undo in its [UndoManager].
+     */
+    fun makeCurrent()
+    fun undo(): Boolean
+    fun redo(): Boolean
+    fun reset()
+}

web/src/main/kotlin/world/phantasmal/web/core/undo/UndoManager.kt

@@ -0,0 +1,51 @@
+package world.phantasmal.web.core.undo
+
+import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.mutableVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.web.core.actions.Action
+
+class UndoManager {
+    private val _current = mutableVal<Undo>(NopUndo)
+
+    val current: Val<Undo> = _current
+
+    val canUndo: Val<Boolean> = current.flatMap { it.canUndo }
+    val canRedo: Val<Boolean> = current.flatMap { it.canRedo }
+    val firstUndo: Val<Action?> = current.flatMap { it.firstUndo }
+    val firstRedo: Val<Action?> = current.flatMap { it.firstRedo }
+
+    fun setCurrent(undo: Undo) {
+        _current.value = undo
+    }
+
+    fun undo(): Boolean =
+        current.value.undo()
+
+    fun redo(): Boolean =
+        current.value.redo()
+
+    fun makeNopCurrent() {
+        setCurrent(NopUndo)
+    }
+
+    private object NopUndo : Undo {
+        override val canUndo = falseVal()
+        override val canRedo = falseVal()
+        override val firstUndo = nullVal()
+        override val firstRedo = nullVal()
+
+        override fun makeCurrent() {
+            // Do nothing.
+        }
+
+        override fun undo(): Boolean = false
+
+        override fun redo(): Boolean = false
+
+        override fun reset() {
+            // Do nothing.
+        }
+    }
+}

web/src/main/kotlin/world/phantasmal/web/core/undo/UndoStack.kt

@@ -0,0 +1,73 @@
+package world.phantasmal.web.core.undo
+
+import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.gt
+import world.phantasmal.observable.value.list.mutableListVal
+import world.phantasmal.observable.value.mutableVal
+import world.phantasmal.web.core.actions.Action
+
+/**
+ * Full-fledged linear undo/redo implementation.
+ */
+class UndoStack(private val manager: UndoManager) : Undo {
+    private val stack = mutableListVal<Action>()
+
+    /**
+     * The index where new actions are inserted. If not equal to the [stack]'s size, points to the
+     * action that will be redone when calling [redo].
+     */
+    private val index = mutableVal(0)
+    private var undoingOrRedoing = false
+
+    override val canUndo: Val<Boolean> = index gt 0
+
+    override val canRedo: Val<Boolean> = stack.map(index) { stack, index -> index < stack.size }
+
+    override val firstUndo: Val<Action?> = index.map { stack.value.getOrNull(it - 1) }
+
+    override val firstRedo: Val<Action?> = index.map { stack.value.getOrNull(it) }
+
+    override fun makeCurrent() {
+        manager.setCurrent(this)
+    }
+
+    fun push(action: Action): Action {
+        if (!undoingOrRedoing) {
+            stack.splice(index.value, stack.value.size - index.value, action)
+            index.value++
+        }
+
+        return action
+    }
+
+    override fun undo(): Boolean {
+        if (undoingOrRedoing || !canUndo.value) return false
+
+        try {
+            undoingOrRedoing = true
+            index.value -= 1
+            stack[index.value].undo()
+        } finally {
+            undoingOrRedoing = false
+            return true
+        }
+    }
+
+    override fun redo(): Boolean {
+        if (undoingOrRedoing || !canRedo.value) return false
+
+        try {
+            undoingOrRedoing = true
+            stack[index.value].execute()
+            index.value += 1
+        } finally {
+            undoingOrRedoing = false
+            return true
+        }
+    }
+
+    override fun reset() {
+        stack.clear()
+        index.value = 0
+    }
+}

web/src/main/kotlin/world/phantasmal/web/core/widgets/DockWidget.kt

@@ -3,10 +3,10 @@ package world.phantasmal.web.core.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
-import world.phantasmal.webui.obj
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.web.externals.goldenLayout.GoldenLayout
 import world.phantasmal.webui.dom.div
+import world.phantasmal.webui.obj
 import world.phantasmal.webui.widgets.Widget
 
 private const val HEADER_HEIGHT = 24
@@ -44,9 +44,9 @@ class DockedWidget(
 
 class DockWidget(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
     private val item: DockedItem,
-) : Widget(scope, hidden) {
+) : Widget(scope, visible) {
     private lateinit var goldenLayout: GoldenLayout
 
     init {

web/src/main/kotlin/world/phantasmal/web/core/widgets/RendererWidget.kt

@@ -21,11 +21,11 @@ class RendererWidget(
 
             observeResize()
 
-            observe(selfOrAncestorHidden) { hidden ->
-                if (hidden) {
-                    renderer.stopRendering()
-                } else {
+            observe(selfOrAncestorVisible) { visible ->
+                if (visible) {
                     renderer.startRendering()
+                } else {
+                    renderer.stopRendering()
                 }
             }
 

web/src/main/kotlin/world/phantasmal/web/core/widgets/UnavailableWidget.kt

@@ -3,21 +3,21 @@ package world.phantasmal.web.core.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.trueVal
+import world.phantasmal.observable.value.falseVal
 import world.phantasmal.webui.dom.div
 import world.phantasmal.webui.widgets.Label
 import world.phantasmal.webui.widgets.Widget
 
 class UnavailableWidget(
     scope: CoroutineScope,
-    hidden: Val<Boolean>,
+    visible: Val<Boolean>,
     private val message: String,
-) : Widget(scope, hidden) {
+) : Widget(scope, visible) {
     override fun Node.createElement() =
         div {
             className = "pw-core-unavailable"
 
-            addWidget(Label(scope, disabled = trueVal(), text = message))
+            addWidget(Label(scope, enabled = falseVal(), text = message))
         }
 
     companion object {

web/src/main/kotlin/world/phantasmal/web/questEditor/QuestEditor.kt

@@ -6,6 +6,7 @@ import org.w3c.dom.HTMLCanvasElement
 import world.phantasmal.web.core.PwTool
 import world.phantasmal.web.core.PwToolType
 import world.phantasmal.web.core.loading.AssetLoader
+import world.phantasmal.web.core.stores.UiStore
 import world.phantasmal.web.externals.babylon.Engine
 import world.phantasmal.web.questEditor.controllers.NpcCountsController
 import world.phantasmal.web.questEditor.controllers.QuestEditorToolbarController
@@ -13,9 +14,9 @@ import world.phantasmal.web.questEditor.controllers.QuestInfoController
 import world.phantasmal.web.questEditor.loading.AreaAssetLoader
 import world.phantasmal.web.questEditor.loading.EntityAssetLoader
 import world.phantasmal.web.questEditor.loading.QuestLoader
-import world.phantasmal.web.questEditor.rendering.UserInputManager
 import world.phantasmal.web.questEditor.rendering.QuestEditorMeshManager
 import world.phantasmal.web.questEditor.rendering.QuestRenderer
+import world.phantasmal.web.questEditor.rendering.UserInputManager
 import world.phantasmal.web.questEditor.stores.AreaStore
 import world.phantasmal.web.questEditor.stores.QuestEditorStore
 import world.phantasmal.web.questEditor.widgets.*
@@ -24,6 +25,7 @@ import world.phantasmal.webui.widgets.Widget
 
 class QuestEditor(
     private val assetLoader: AssetLoader,
+    private val uiStore: UiStore,
     private val createEngine: (HTMLCanvasElement) -> Engine,
 ) : DisposableContainer(), PwTool {
     override val toolType = PwToolType.QuestEditor
@@ -40,11 +42,14 @@ class QuestEditor(
 
         // Stores
         val areaStore = addDisposable(AreaStore(scope, areaAssetLoader))
-        val questEditorStore = addDisposable(QuestEditorStore(scope, areaStore))
+        val questEditorStore = addDisposable(QuestEditorStore(scope, uiStore, areaStore))
 
         // Controllers
-        val toolbarController =
-            addDisposable(QuestEditorToolbarController(questLoader, areaStore, questEditorStore))
+        val toolbarController = addDisposable(QuestEditorToolbarController(
+            questLoader,
+            areaStore,
+            questEditorStore,
+        ))
         val questInfoController = addDisposable(QuestInfoController(questEditorStore))
         val npcCountsController = addDisposable(NpcCountsController(questEditorStore))
 

web/src/main/kotlin/world/phantasmal/web/questEditor/QuestRunner.kt

@@ -0,0 +1,12 @@
+package world.phantasmal.web.questEditor
+
+import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.falseVal
+
+/**
+ * Orchestrates everything related to emulating a quest run. Drives a [VirtualMachine] and
+ * delegates to [Debugger].
+ */
+class QuestRunner {
+    val running: Val<Boolean> = falseVal()
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/TranslateEntityAction.kt

@@ -0,0 +1,42 @@
+package world.phantasmal.web.questEditor.actions
+
+import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.externals.babylon.Vector3
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.models.SectionModel
+
+class TranslateEntityAction(
+    private val setSelectedEntity: (QuestEntityModel<*, *>) -> Unit,
+    private val entity: QuestEntityModel<*, *>,
+    private val oldSection: SectionModel?,
+    private val newSection: SectionModel?,
+    private val oldPosition: Vector3,
+    private val newPosition: Vector3,
+    private val world: Boolean,
+) : Action {
+    override val description: String = "Move ${entity.type.simpleName}"
+
+    override fun execute() {
+        setSelectedEntity(entity)
+
+        newSection?.let(entity::setSection)
+
+        if (world) {
+            entity.setWorldPosition(newPosition)
+        } else {
+            entity.setPosition(newPosition)
+        }
+    }
+
+    override fun undo() {
+        setSelectedEntity(entity)
+
+        oldSection?.let(entity::setSection)
+
+        if (world) {
+            entity.setWorldPosition(oldPosition)
+        } else {
+            entity.setPosition(oldPosition)
+        }
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/NpcCountsController.kt

@@ -2,13 +2,14 @@ package world.phantasmal.web.questEditor.controllers
 
 import world.phantasmal.lib.fileFormats.quest.NpcType
 import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.isNull
 import world.phantasmal.observable.value.list.emptyListVal
 import world.phantasmal.web.questEditor.models.QuestNpcModel
 import world.phantasmal.web.questEditor.stores.QuestEditorStore
 import world.phantasmal.webui.controllers.Controller
 
 class NpcCountsController(store: QuestEditorStore) : Controller() {
-    val unavailable: Val<Boolean> = store.currentQuest.map { it == null }
+    val unavailable: Val<Boolean> = store.currentQuest.isNull()
 
     val npcCounts: Val<List<NameWithCount>> = store.currentQuest
         .flatMap { it?.npcs ?: emptyListVal() }

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/QuestEditorToolbarController.kt

@@ -9,9 +9,8 @@ import world.phantasmal.lib.fileFormats.quest.Episode
 import world.phantasmal.lib.fileFormats.quest.Quest
 import world.phantasmal.lib.fileFormats.quest.parseBinDatToQuest
 import world.phantasmal.lib.fileFormats.quest.parseQstToQuest
-import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.mutableVal
-import world.phantasmal.observable.value.value
+import world.phantasmal.observable.value.*
+import world.phantasmal.web.core.undo.UndoManager
 import world.phantasmal.web.questEditor.loading.QuestLoader
 import world.phantasmal.web.questEditor.models.AreaModel
 import world.phantasmal.web.questEditor.stores.AreaStore
@@ -32,11 +31,31 @@ class QuestEditorToolbarController(
     private val _resultDialogVisible = mutableVal(false)
     private val _result = mutableVal<PwResult<*>?>(null)
 
+    // Result
+
     val resultDialogVisible: Val<Boolean> = _resultDialogVisible
     val result: Val<PwResult<*>?> = _result
 
-    // Ensure the areas list is updated when entities are added or removed (the count in the
-    // label should update).
+    // Undo
+
+    val undoTooltip: Val<String> = questEditorStore.firstUndo.map { action ->
+        (action?.let { "Undo \"${action.description}\"" } ?: "Nothing to undo") + " (Ctrl-Z)"
+    }
+
+    val undoEnabled: Val<Boolean> = questEditorStore.canUndo
+
+    // Redo
+
+    val redoTooltip: Val<String> = questEditorStore.firstRedo.map { action ->
+        (action?.let { "Redo \"${action.description}\"" } ?: "Nothing to redo") + " (Ctrl-Shift-Z)"
+    }
+
+    val redoEnabled: Val<Boolean> = questEditorStore.canRedo
+
+    // Areas
+
+    // Ensure the areas list is updated when entities are added or removed (the count in the label
+    // should update).
     val areas: Val<List<AreaAndLabel>> = questEditorStore.currentQuest.flatMap { quest ->
         quest?.let {
             quest.entitiesPerArea.map { entitiesPerArea ->
@@ -47,15 +66,12 @@ class QuestEditorToolbarController(
             }
         } ?: value(emptyList())
     }
+
     val currentArea: Val<AreaAndLabel?> = areas.map(questEditorStore.currentArea) { areas, area ->
         areas.find { it.area == area }
     }
-    val areaSelectDisabled: Val<Boolean>
 
-    init {
-        val noQuestLoaded = questEditorStore.currentQuest.map { it == null }
-        areaSelectDisabled = noQuestLoaded
-    }
+    val areaSelectEnabled: Val<Boolean> = questEditorStore.currentQuest.isNotNull()
 
     suspend fun createNewQuest(episode: Episode) {
         questEditorStore.setCurrentQuest(
@@ -107,6 +123,14 @@ class QuestEditorToolbarController(
         }
     }
 
+    fun undo() {
+        questEditorStore.undo()
+    }
+
+    fun redo() {
+        questEditorStore.redo()
+    }
+
     fun setCurrentArea(areaAndLabel: AreaAndLabel) {
         questEditorStore.setCurrentArea(areaAndLabel.area)
     }

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/QuestInfoController.kt

@@ -1,13 +1,14 @@
 package world.phantasmal.web.questEditor.controllers
 
 import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.isNull
 import world.phantasmal.observable.value.value
 import world.phantasmal.web.questEditor.stores.QuestEditorStore
 import world.phantasmal.webui.controllers.Controller
 
 class QuestInfoController(store: QuestEditorStore) : Controller() {
-    val unavailable: Val<Boolean> = store.currentQuest.map { it == null }
-    val disabled: Val<Boolean> = store.questEditingDisabled
+    val unavailable: Val<Boolean> = store.currentQuest.isNull()
+    val enabled: Val<Boolean> = store.questEditingEnabled
 
     val episode: Val<String> = store.currentQuest.map { it?.episode?.name ?: "" }
     val id: Val<Int> = store.currentQuest.flatMap { it?.id ?: value(0) }

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/EntityMeshManager.kt

@@ -4,6 +4,7 @@ import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.launch
 import mu.KotlinLogging
 import world.phantasmal.observable.value.Val
+import world.phantasmal.observable.value.isNotNull
 import world.phantasmal.web.externals.babylon.AbstractMesh
 import world.phantasmal.web.externals.babylon.TransformNode
 import world.phantasmal.web.questEditor.loading.EntityAssetLoader
@@ -162,7 +163,7 @@ class EntityMeshManager(
                         sectionInitialized && (sWave == null || sWave == entityWave)
                     }
             } else {
-                isVisible = entity.section.map { section -> section != null }
+                isVisible = entity.section.isNotNull()
 
                 if (entity is QuestObjectModel) {
                     addDisposable(entity.model.observe(callNow = false) {

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/UserInputManager.kt

@@ -157,6 +157,24 @@ private class StateContext(
         }
     }
 
+    fun finalizeTranslation(
+        entity: QuestEntityModel<*, *>,
+        oldSection: SectionModel?,
+        newSection: SectionModel?,
+        oldPosition: Vector3,
+        newPosition: Vector3,
+        world: Boolean,
+    ) {
+        questEditorStore.translateEntity(
+            entity,
+            oldSection,
+            newSection,
+            oldPosition,
+            newPosition,
+            world
+        )
+    }
+
     /**
      * If the drag-adjusted pointer is over the ground, translate an entity horizontally across the
      * ground. Otherwise translate the entity over the horizontal plane that intersects its origin.
@@ -426,22 +444,14 @@ private class TranslationState(
                 ctx.renderer.enableCameraControls()
 
                 if (!cancelled && event.movedSinceLastPointerDown) {
-                    // TODO
-//                    questEditorStore.undo
-//                        .push(
-//                            new TranslateEntityAction (
-//                                    this.questEditorStore,
-//                            this.entity,
-//                            this.initialSection,
-//                            this.entity.section.
-//                            val ,
-//                                    this.initial_position,
-//                            this.entity.world_position.
-//                            val ,
-//                            true,
-//                        ),
-//                    )
-//                    .redo()
+                    ctx.finalizeTranslation(
+                        entity,
+                        initialSection,
+                        entity.section.value,
+                        initialPosition,
+                        entity.worldPosition.value,
+                        true,
+                    )
                 }
 
                 IdleState(ctx, entityManipulationEnabled = true)

web/src/main/kotlin/world/phantasmal/web/questEditor/stores/QuestEditorStore.kt

@@ -2,28 +2,75 @@ package world.phantasmal.web.questEditor.stores
 
 import kotlinx.coroutines.CoroutineScope
 import mu.KotlinLogging
-import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.mutableVal
+import world.phantasmal.observable.value.*
+import world.phantasmal.web.core.PwToolType
+import world.phantasmal.web.core.stores.UiStore
+import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.undo.UndoManager
+import world.phantasmal.web.core.undo.UndoStack
+import world.phantasmal.web.externals.babylon.Vector3
+import world.phantasmal.web.questEditor.QuestRunner
+import world.phantasmal.web.questEditor.actions.TranslateEntityAction
 import world.phantasmal.web.questEditor.models.*
 import world.phantasmal.webui.stores.Store
 
 private val logger = KotlinLogging.logger {}
 
-class QuestEditorStore(scope: CoroutineScope, private val areaStore: AreaStore) : Store(scope) {
+class QuestEditorStore(
+    scope: CoroutineScope,
+    private val uiStore: UiStore,
+    private val areaStore: AreaStore,
+) : Store(scope) {
     private val _currentQuest = mutableVal<QuestModel?>(null)
     private val _currentArea = mutableVal<AreaModel?>(null)
     private val _selectedWave = mutableVal<WaveModel?>(null)
     private val _selectedEntity = mutableVal<QuestEntityModel<*, *>?>(null)
 
+    private val undoManager = UndoManager()
+    private val mainUndo = UndoStack(undoManager)
+
+    val runner = QuestRunner()
     val currentQuest: Val<QuestModel?> = _currentQuest
     val currentArea: Val<AreaModel?> = _currentArea
     val selectedWave: Val<WaveModel?> = _selectedWave
     val selectedEntity: Val<QuestEntityModel<*, *>?> = _selectedEntity
 
-    // TODO: Take into account whether we're debugging or not.
-    val questEditingDisabled: Val<Boolean> = currentQuest.map { it == null }
+    val questEditingEnabled: Val<Boolean> = currentQuest.isNotNull() and !runner.running
+    val canUndo: Val<Boolean> = questEditingEnabled and undoManager.canUndo
+    val firstUndo: Val<Action?> = undoManager.firstUndo
+    val canRedo: Val<Boolean> = questEditingEnabled and undoManager.canRedo
+    val firstRedo: Val<Action?> = undoManager.firstRedo
+
+    init {
+        observe(uiStore.currentTool) { tool ->
+            if (tool == PwToolType.QuestEditor) {
+                mainUndo.makeCurrent()
+            }
+        }
+    }
+
+    fun makeMainUndoCurrent() {
+        mainUndo.makeCurrent()
+    }
+
+    fun undo() {
+        require(canUndo.value) { "Can't undo at the moment." }
+        undoManager.undo()
+    }
+
+    fun redo() {
+        require(canRedo.value) { "Can't redo at the moment." }
+        undoManager.redo()
+    }
 
     suspend fun setCurrentQuest(quest: QuestModel?) {
+        mainUndo.reset()
+
+        // TODO: Stop runner.
+
+        _selectedEntity.value = null
+        _selectedWave.value = null
+
         if (quest == null) {
             _currentArea.value = null
             _currentQuest.value = null
@@ -80,4 +127,23 @@ class QuestEditorStore(scope: CoroutineScope, private val areaStore: AreaStore)
 
         _selectedEntity.value = entity
     }
+
+    fun translateEntity(
+        entity: QuestEntityModel<*, *>,
+        oldSection: SectionModel?,
+        newSection: SectionModel?,
+        oldPosition: Vector3,
+        newPosition: Vector3,
+        world: Boolean,
+    ) {
+        mainUndo.push(TranslateEntityAction(
+            ::setSelectedEntity,
+            entity,
+            oldSection,
+            newSection,
+            oldPosition,
+            newPosition,
+            world,
+        )).execute()
+    }
 }

web/src/main/kotlin/world/phantasmal/web/questEditor/widgets/NpcCountsWidget.kt

@@ -2,7 +2,6 @@ package world.phantasmal.web.questEditor.widgets
 
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
-import world.phantasmal.observable.value.not
 import world.phantasmal.web.core.widgets.UnavailableWidget
 import world.phantasmal.web.questEditor.controllers.NpcCountsController
 import world.phantasmal.webui.dom.*
@@ -28,7 +27,7 @@ class NpcCountsWidget(
             }
             addChild(UnavailableWidget(
                 scope,
-                hidden = !ctrl.unavailable,
+                visible = ctrl.unavailable,
                 message = "No quest loaded."
             ))
         }

web/src/main/kotlin/world/phantasmal/web/questEditor/widgets/QuestEditorToolbarWidget.kt

@@ -24,7 +24,7 @@ class QuestEditorToolbarWidget(
                         scope,
                         text = "New quest",
                         iconLeft = Icon.NewFile,
-                        onClick = { scope.launch { ctrl.createNewQuest(Episode.I) } }
+                        onClick = { scope.launch { ctrl.createNewQuest(Episode.I) } },
                     ),
                     FileButton(
                         scope,
@@ -32,15 +32,31 @@ class QuestEditorToolbarWidget(
                         iconLeft = Icon.File,
                         accept = ".bin, .dat, .qst",
                         multiple = true,
-                        filesSelected = { files -> scope.launch { ctrl.openFiles(files) } }
+                        filesSelected = { files -> scope.launch { ctrl.openFiles(files) } },
+                    ),
+                    Button(
+                        scope,
+                        text = "Undo",
+                        iconLeft = Icon.Undo,
+                        enabled = ctrl.undoEnabled,
+                        tooltip = ctrl.undoTooltip,
+                        onClick = { ctrl.undo() },
+                    ),
+                    Button(
+                        scope,
+                        text = "Redo",
+                        iconLeft = Icon.Redo,
+                        enabled = ctrl.redoEnabled,
+                        tooltip = ctrl.redoTooltip,
+                        onClick = { ctrl.redo() },
                     ),
                     Select(
                         scope,
-                        disabled = ctrl.areaSelectDisabled,
+                        enabled = ctrl.areaSelectEnabled,
                         itemsVal = ctrl.areas,
                         itemToString = { it.label },
                         selectedVal = ctrl.currentArea,
-                        onSelect = ctrl::setCurrentArea
+                        onSelect = ctrl::setCurrentArea,
                     )
                 )
             ))

web/src/main/kotlin/world/phantasmal/web/questEditor/widgets/QuestInfoWidget.kt

@@ -2,7 +2,6 @@ package world.phantasmal.web.questEditor.widgets
 
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
-import world.phantasmal.observable.value.not
 import world.phantasmal.web.core.widgets.UnavailableWidget
 import world.phantasmal.web.questEditor.controllers.QuestInfoController
 import world.phantasmal.webui.dom.*
@@ -14,7 +13,7 @@ import world.phantasmal.webui.widgets.Widget
 class QuestInfoWidget(
     scope: CoroutineScope,
     private val ctrl: QuestInfoController,
-) : Widget(scope, disabled = ctrl.disabled) {
+) : Widget(scope, enabled = ctrl.enabled) {
     override fun Node.createElement() =
         div {
             className = "pw-quest-editor-quest-info"
@@ -32,7 +31,7 @@ class QuestInfoWidget(
                     td {
                         addChild(IntInput(
                             this@QuestInfoWidget.scope,
-                            disabled = ctrl.disabled,
+                            enabled = ctrl.enabled,
                             valueVal = ctrl.id,
                             min = 0,
                             step = 1,
@@ -44,7 +43,7 @@ class QuestInfoWidget(
                     td {
                         addChild(TextInput(
                             this@QuestInfoWidget.scope,
-                            disabled = ctrl.disabled,
+                            enabled = ctrl.enabled,
                             valueVal = ctrl.name,
                             maxLength = 32,
                         ))
@@ -61,7 +60,7 @@ class QuestInfoWidget(
                         colSpan = 2
                         addChild(TextArea(
                             this@QuestInfoWidget.scope,
-                            disabled = ctrl.disabled,
+                            enabled = ctrl.enabled,
                             valueVal = ctrl.shortDescription,
                             maxLength = 128,
                             fontFamily = "\"Courier New\", monospace",
@@ -81,7 +80,7 @@ class QuestInfoWidget(
                         colSpan = 2
                         addChild(TextArea(
                             this@QuestInfoWidget.scope,
-                            disabled = ctrl.disabled,
+                            enabled = ctrl.enabled,
                             valueVal = ctrl.longDescription,
                             maxLength = 288,
                             fontFamily = "\"Courier New\", monospace",
@@ -93,7 +92,7 @@ class QuestInfoWidget(
             }
             addChild(UnavailableWidget(
                 scope,
-                hidden = !ctrl.unavailable,
+                visible = ctrl.unavailable,
                 message = "No quest loaded."
             ))
         }

web/src/test/kotlin/world/phantasmal/web/questEditor/QuestEditorTests.kt

@@ -8,7 +8,7 @@ class QuestEditorTests : WebTestSuite() {
     @Test
     fun initialization_and_shutdown_should_succeed_without_throwing() = test {
         val questEditor = disposer.add(
-            QuestEditor(components.assetLoader, createEngine = { Engine(it) })
+            QuestEditor(components.assetLoader, components.uiStore, createEngine = { Engine(it) })
         )
         disposer.add(questEditor.initialize(scope))
     }

web/src/test/kotlin/world/phantasmal/web/questEditor/controllers/NpcCountsControllerTests.kt

@@ -12,7 +12,7 @@ import kotlin.test.assertTrue
 
 class NpcCountsControllerTests : WebTestSuite() {
     @Test
-    fun exposes_correct_model_before_and_after_a_quest_is_loaded() = test {
+    fun exposes_correct_model_before_and_after_a_quest_is_loaded() = asyncTest {
         val store = components.questEditorStore
         val ctrl = disposer.add(NpcCountsController(store))
 

web/src/test/kotlin/world/phantasmal/web/questEditor/controllers/QuestEditorToolbarControllerTests.kt

@@ -3,11 +3,13 @@ package world.phantasmal.web.questEditor.controllers
 import org.w3c.files.File
 import world.phantasmal.core.Failure
 import world.phantasmal.core.Severity
+import world.phantasmal.lib.fileFormats.quest.Episode
+import world.phantasmal.lib.fileFormats.quest.NpcType
+import world.phantasmal.web.externals.babylon.Vector3
 import world.phantasmal.web.test.WebTestSuite
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertNull
-import kotlin.test.assertTrue
+import world.phantasmal.web.test.createQuestModel
+import world.phantasmal.web.test.createQuestNpcModel
+import kotlin.test.*
 
 class QuestEditorToolbarControllerTests : WebTestSuite() {
     @Test
@@ -15,7 +17,7 @@ class QuestEditorToolbarControllerTests : WebTestSuite() {
         val ctrl = disposer.add(QuestEditorToolbarController(
             components.questLoader,
             components.areaStore,
-            components.questEditorStore
+            components.questEditorStore,
         ))
 
         assertNull(ctrl.result.value)
@@ -29,7 +31,84 @@ class QuestEditorToolbarControllerTests : WebTestSuite() {
         assertEquals(Severity.Error, result.problems.first().severity)
         assertEquals(
             "Please select a .qst file or one .bin and one .dat file.",
-            result.problems.first().uiMessage
+            result.problems.first().uiMessage,
         )
     }
+
+    @Test
+    fun undo_state_changes_correctly() = asyncTest {
+        val ctrl = disposer.add(QuestEditorToolbarController(
+            components.questLoader,
+            components.areaStore,
+            components.questEditorStore,
+        ))
+        components.questEditorStore.makeMainUndoCurrent()
+        val nothingToUndo = "Nothing to undo (Ctrl-Z)"
+        val nothingToRedo = "Nothing to redo (Ctrl-Shift-Z)"
+
+        // No quest loaded.
+
+        assertEquals(nothingToUndo, ctrl.undoTooltip.value)
+        assertFalse(ctrl.undoEnabled.value)
+
+        assertEquals(nothingToRedo, ctrl.redoTooltip.value)
+        assertFalse(ctrl.redoEnabled.value)
+
+        // Load quest.
+        val npc = createQuestNpcModel(NpcType.Scientist, Episode.I)
+        components.questEditorStore.setCurrentQuest(createQuestModel(npcs= listOf(npc)))
+
+        assertEquals(nothingToUndo, ctrl.undoTooltip.value)
+        assertFalse(ctrl.undoEnabled.value)
+
+        assertEquals(nothingToRedo, ctrl.redoTooltip.value)
+        assertFalse(ctrl.redoEnabled.value)
+
+        // Add an action to the undo stack.
+        components.questEditorStore.translateEntity(
+            npc,
+            null,
+            null,
+            Vector3.Zero(),
+            Vector3.Up(),
+            true,
+        )
+
+        assertEquals("Undo \"Move Scientist\" (Ctrl-Z)", ctrl.undoTooltip.value)
+        assertTrue(ctrl.undoEnabled.value)
+
+        assertEquals(nothingToRedo, ctrl.redoTooltip.value)
+        assertFalse(ctrl.redoEnabled.value)
+
+        // Undo the previous action.
+        ctrl.undo()
+
+        assertEquals(nothingToUndo, ctrl.undoTooltip.value)
+        assertFalse(ctrl.undoEnabled.value)
+
+        assertEquals("Redo \"Move Scientist\" (Ctrl-Shift-Z)", ctrl.redoTooltip.value)
+        assertTrue(ctrl.redoEnabled.value)
+    }
+
+    @Test
+    fun area_state_changes_correctly() = asyncTest {
+        val ctrl = disposer.add(QuestEditorToolbarController(
+            components.questLoader,
+            components.areaStore,
+            components.questEditorStore,
+        ))
+
+        // No quest loaded.
+
+        assertTrue(ctrl.areas.value.isEmpty())
+        assertNull(ctrl.currentArea.value)
+        assertFalse(ctrl.areaSelectEnabled.value)
+
+        // Load quest.
+        components.questEditorStore.setCurrentQuest(createQuestModel())
+
+        assertTrue(ctrl.areas.value.isNotEmpty())
+        assertNotNull(ctrl.currentArea.value)
+        assertTrue(ctrl.areaSelectEnabled.value)
+    }
 }

web/src/test/kotlin/world/phantasmal/web/questEditor/controllers/QuestInfoControllerTests.kt

@@ -10,12 +10,12 @@ import kotlin.test.assertTrue
 
 class QuestInfoControllerTests : WebTestSuite() {
     @Test
-    fun exposes_correct_model_before_and_after_a_quest_is_loaded() = test {
+    fun exposes_correct_model_before_and_after_a_quest_is_loaded() = asyncTest {
         val store = components.questEditorStore
         val ctrl = disposer.add(QuestInfoController(store))
 
         assertTrue(ctrl.unavailable.value)
-        assertTrue(ctrl.disabled.value)
+        assertFalse(ctrl.enabled.value)
 
         store.setCurrentQuest(createQuestModel(
             id = 25,
@@ -26,7 +26,7 @@ class QuestInfoControllerTests : WebTestSuite() {
         ))
 
         assertFalse(ctrl.unavailable.value)
-        assertFalse(ctrl.disabled.value)
+        assertTrue(ctrl.enabled.value)
         assertEquals("II", ctrl.episode.value)
         assertEquals(25, ctrl.id.value)
         assertEquals("A Quest", ctrl.name.value)

web/src/test/kotlin/world/phantasmal/web/questEditor/undo/UndoStackTests.kt

@@ -0,0 +1,115 @@
+package world.phantasmal.web.questEditor.undo
+
+import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.undo.UndoManager
+import world.phantasmal.web.core.undo.UndoStack
+import world.phantasmal.web.test.WebTestSuite
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class UndoStackTests : WebTestSuite() {
+    @Test
+    fun simple_properties_and_invariants() {
+        val stack = UndoStack(UndoManager())
+
+        assertFalse(stack.canUndo.value)
+        assertFalse(stack.canRedo.value)
+
+        stack.push(DummyAction())
+        stack.push(DummyAction())
+        stack.push(DummyAction())
+
+        assertTrue(stack.canUndo.value)
+        assertFalse(stack.canRedo.value)
+
+        stack.undo()
+
+        assertTrue(stack.canUndo.value)
+        assertTrue(stack.canRedo.value)
+
+        stack.undo()
+        stack.undo()
+
+        assertFalse(stack.canUndo.value)
+        assertTrue(stack.canRedo.value)
+    }
+
+    @Test
+    fun undo() {
+        val stack = UndoStack(UndoManager())
+
+        var value = 3
+
+        stack.push(DummyAction(execute = { value = 7 }, undo = { value = 3 })).execute()
+        stack.push(DummyAction(execute = { value = 13 }, undo = { value = 7 })).execute()
+
+        assertEquals(13, value)
+
+        assertTrue(stack.undo())
+        assertEquals(7, value)
+
+        assertTrue(stack.undo())
+        assertEquals(3, value)
+
+        assertFalse(stack.undo())
+        assertEquals(3, value)
+    }
+
+    @Test
+    fun redo() {
+        val stack = UndoStack(UndoManager())
+
+        var value = 3
+
+        stack.push(DummyAction(execute = { value = 7 }, undo = { value = 3 })).execute()
+        stack.push(DummyAction(execute = { value = 13 }, undo = { value = 7 })).execute()
+
+        stack.undo()
+        stack.undo()
+
+        assertEquals(3, value)
+
+        assertTrue(stack.redo())
+        assertEquals(7, value)
+
+        assertTrue(stack.redo())
+        assertEquals(13, value)
+
+        assertFalse(stack.redo())
+        assertEquals(13, value)
+    }
+
+    @Test
+    fun push_then_undo_then_push_again() {
+        val stack = UndoStack(UndoManager())
+
+        var value = 3
+
+        stack.push(DummyAction(execute = { value = 7 }, undo = { value = 3 })).execute()
+
+        stack.undo()
+
+        assertEquals(3, value)
+
+        stack.push(DummyAction(execute = { value = 13 }, undo = { value = 7 })).execute()
+
+        assertEquals(13, value)
+    }
+
+    private class DummyAction(
+        private val execute: () -> Unit = {},
+        private val undo: () -> Unit = {},
+    ) : Action {
+        override val description: String = "Dummy action"
+
+        override fun execute() {
+            execute.invoke()
+        }
+
+        override fun undo() {
+            undo.invoke()
+        }
+    }
+}

web/src/test/kotlin/world/phantasmal/web/test/TestComponents.kt

@@ -8,6 +8,8 @@ import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.disposable
 import world.phantasmal.testUtils.TestContext
 import world.phantasmal.web.core.loading.AssetLoader
+import world.phantasmal.web.core.stores.ApplicationUrl
+import world.phantasmal.web.core.stores.UiStore
 import world.phantasmal.web.externals.babylon.Engine
 import world.phantasmal.web.externals.babylon.Scene
 import world.phantasmal.web.questEditor.loading.AreaAssetLoader
@@ -33,6 +35,8 @@ class TestComponents(private val ctx: TestContext) {
         }
     }
 
+    var applicationUrl: ApplicationUrl by default { TestApplicationUrl("") }
+
     // Babylon.js
 
     var scene: Scene by default { Scene(Engine(null)) }
@@ -49,10 +53,12 @@ class TestComponents(private val ctx: TestContext) {
 
     // Stores
 
+    var uiStore: UiStore by default { UiStore(ctx.scope, applicationUrl) }
+
     var areaStore: AreaStore by default { AreaStore(ctx.scope, areaAssetLoader) }
 
     var questEditorStore: QuestEditorStore by default {
-        QuestEditorStore(ctx.scope, areaStore)
+        QuestEditorStore(ctx.scope, uiStore, areaStore)
     }
 
     private fun <T> default(defaultValue: () -> T) = LazyDefault {

webui/src/main/kotlin/world/phantasmal/webui/controllers/TabController.kt

@@ -17,5 +17,5 @@ open class TabController<T : Tab>(val tabs: List<T>) : Controller() {
         _activeTab.value = tab
     }
 
-    open fun hiddenChanged(hidden: Boolean) {}
+    open fun visibleChanged(visible: Boolean) {}
 }

webui/src/main/kotlin/world/phantasmal/webui/widgets/Button.kt

@@ -5,7 +5,8 @@ import org.w3c.dom.Node
 import org.w3c.dom.events.KeyboardEvent
 import org.w3c.dom.events.MouseEvent
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.dom.Icon
 import world.phantasmal.webui.dom.button
 import world.phantasmal.webui.dom.icon
@@ -13,8 +14,9 @@ import world.phantasmal.webui.dom.span
 
 open class Button(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     private val text: String? = null,
     private val textVal: Val<String>? = null,
     private val iconLeft: Icon? = null,
@@ -25,7 +27,7 @@ open class Button(
     private val onKeyDown: ((KeyboardEvent) -> Unit)? = null,
     private val onKeyUp: ((KeyboardEvent) -> Unit)? = null,
     private val onKeyPress: ((KeyboardEvent) -> Unit)? = null,
-) : Control(scope, hidden, disabled) {
+) : Control(scope, visible, enabled, tooltip) {
     override fun Node.createElement() =
         button {
             className = "pw-button"

webui/src/main/kotlin/world/phantasmal/webui/widgets/Control.kt

@@ -2,7 +2,8 @@ package world.phantasmal.webui.widgets
 
 import kotlinx.coroutines.CoroutineScope
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 
 /**
  * Represents all widgets that allow for user interaction such as buttons, text inputs, combo boxes,
@@ -10,7 +11,7 @@ import world.phantasmal.observable.value.falseVal
  */
 abstract class Control(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
-) : Widget(scope, hidden, disabled, tooltip)
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
+) : Widget(scope, visible, enabled, tooltip)

webui/src/main/kotlin/world/phantasmal/webui/widgets/DoubleInput.kt

@@ -3,15 +3,16 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.HTMLInputElement
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 import kotlin.math.pow
 import kotlin.math.round
 
 class DoubleInput(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     label: String? = null,
     labelVal: Val<String>? = null,
     preferredLabelPosition: LabelPosition = LabelPosition.Before,
@@ -21,8 +22,8 @@ class DoubleInput(
     roundTo: Int = 2,
 ) : NumberInput<Double>(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,

webui/src/main/kotlin/world/phantasmal/webui/widgets/FileButton.kt

@@ -5,13 +5,16 @@ import org.w3c.dom.HTMLElement
 import org.w3c.files.File
 import world.phantasmal.observable.value.Val
 import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.dom.Icon
 import world.phantasmal.webui.openFiles
 
 class FileButton(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     text: String? = null,
     textVal: Val<String>? = null,
     iconLeft: Icon? = null,
@@ -19,7 +22,7 @@ class FileButton(
     private val accept: String = "",
     private val multiple: Boolean = false,
     private val filesSelected: ((List<File>) -> Unit)? = null,
-) : Button(scope, hidden, disabled, text, textVal, iconLeft, iconRight) {
+) : Button(scope, visible, enabled, tooltip, text, textVal, iconLeft, iconRight) {
     override fun interceptElement(element: HTMLElement) {
         element.classList.add("pw-file-button")
 

webui/src/main/kotlin/world/phantasmal/webui/widgets/Input.kt

@@ -9,9 +9,9 @@ import world.phantasmal.webui.dom.span
 
 abstract class Input<T>(
     scope: CoroutineScope,
-    hidden: Val<Boolean>,
-    disabled: Val<Boolean>,
-    tooltip: String?,
+    visible: Val<Boolean>,
+    enabled: Val<Boolean>,
+    tooltip: Val<String?>,
     label: String?,
     labelVal: Val<String>?,
     preferredLabelPosition: LabelPosition,
@@ -27,8 +27,8 @@ abstract class Input<T>(
     private val step: Int?,
 ) : LabelledControl(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,
@@ -42,7 +42,7 @@ abstract class Input<T>(
                 classList.add("pw-input-inner", inputClassName)
                 type = inputType
 
-                observe(this@Input.disabled) { disabled = it }
+                observe(this@Input.enabled) { disabled = !it }
 
                 onchange = { onChange(getInputValue(this)) }
 

webui/src/main/kotlin/world/phantasmal/webui/widgets/IntInput.kt

@@ -3,13 +3,14 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.HTMLInputElement
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 
 class IntInput(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     label: String? = null,
     labelVal: Val<String>? = null,
     preferredLabelPosition: LabelPosition = LabelPosition.Before,
@@ -21,8 +22,8 @@ class IntInput(
     step: Int? = null,
 ) : NumberInput<Int>(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,

webui/src/main/kotlin/world/phantasmal/webui/widgets/Label.kt

@@ -3,17 +3,17 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.dom.label
 
 class Label(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
     private val text: String? = null,
     private val textVal: Val<String>? = null,
     private val htmlFor: String? = null,
-) : Widget(scope, hidden, disabled) {
+) : Widget(scope, visible, enabled) {
     override fun Node.createElement() =
         label {
             className = "pw-label"

webui/src/main/kotlin/world/phantasmal/webui/widgets/LabelledControl.kt

@@ -10,13 +10,13 @@ enum class LabelPosition {
 
 abstract class LabelledControl(
     scope: CoroutineScope,
-    hidden: Val<Boolean>,
-    disabled: Val<Boolean>,
-    tooltip: String? = null,
+    visible: Val<Boolean>,
+    enabled: Val<Boolean>,
+    tooltip: Val<String?>,
     label: String?,
     labelVal: Val<String>?,
     val preferredLabelPosition: LabelPosition,
-) : Control(scope, hidden, disabled, tooltip) {
+) : Control(scope, visible, enabled, tooltip) {
     val label: Label? by lazy {
         if (label == null && labelVal == null) {
             null
@@ -28,7 +28,7 @@ abstract class LabelledControl(
                 element.id = id
             }
 
-            Label(scope, hidden, disabled, label, labelVal, htmlFor = id)
+            Label(scope, visible, enabled, label, labelVal, htmlFor = id)
         }
     }
 

webui/src/main/kotlin/world/phantasmal/webui/widgets/LazyLoader.kt

@@ -3,23 +3,23 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.dom.div
 
 class LazyLoader(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
     private val createWidget: (CoroutineScope) -> Widget,
-) : Widget(scope, hidden, disabled) {
+) : Widget(scope, visible, enabled) {
     private var initialized = false
 
     override fun Node.createElement() =
         div {
             className = "pw-lazy-loader"
 
-            observe(this@LazyLoader.hidden) { h ->
-                if (!h && !initialized) {
+            observe(this@LazyLoader.visible) { v ->
+                if (v && !initialized) {
                     initialized = true
                     addChild(createWidget(scope))
                 }

webui/src/main/kotlin/world/phantasmal/webui/widgets/Menu.kt

@@ -8,7 +8,8 @@ import org.w3c.dom.events.KeyboardEvent
 import org.w3c.dom.events.MouseEvent
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.observable.value.value
 import world.phantasmal.webui.dom.disposableListener
 import world.phantasmal.webui.dom.div
@@ -16,9 +17,9 @@ import world.phantasmal.webui.obj
 
 class Menu<T : Any>(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     items: List<T>? = null,
     itemsVal: Val<List<T>>? = null,
     private val itemToString: (T) -> String = Any::toString,
@@ -26,8 +27,8 @@ class Menu<T : Any>(
     private val onCancel: () -> Unit = {},
 ) : Widget(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
 ) {
     private val items: Val<List<T>> = itemsVal ?: value(items ?: emptyList())
@@ -57,21 +58,21 @@ class Menu<T : Any>(
                 }
             }
 
-            observe(this@Menu.hidden) {
+            observe(this@Menu.visible) {
                 if (it) {
+                    onDocumentMouseDownListener =
+                        disposableListener(document, "mousedown", ::onDocumentMouseDown)
+                } else {
                     onDocumentMouseDownListener?.dispose()
                     onDocumentMouseDownListener = null
                     clearHighlightItem()
 
                     (previouslyFocusedElement as HTMLElement?)?.focus()
-                } else {
-                    onDocumentMouseDownListener =
-                        disposableListener(document, "mousedown", ::onDocumentMouseDown)
                 }
             }
 
-            observe(disabled) {
-                if (it) {
+            observe(enabled) {
+                if (!it) {
                     clearHighlightItem()
                 }
             }
@@ -170,7 +171,7 @@ class Menu<T : Any>(
     private fun highlightItemAt(index: Int) {
         highlightedElement?.classList?.remove("pw-menu-highlighted")
 
-        if (disabled.value) return
+        if (!enabled.value) return
 
         highlightedElement = innerElement.children.item(index)
 
@@ -182,7 +183,7 @@ class Menu<T : Any>(
     }
 
     private fun selectItem(index: Int) {
-        if (disabled.value) return
+        if (!enabled.value) return
 
         items.value.getOrNull(index)?.let(onSelect)
     }

webui/src/main/kotlin/world/phantasmal/webui/widgets/NumberInput.kt

@@ -5,9 +5,9 @@ import world.phantasmal.observable.value.Val
 
 abstract class NumberInput<T : Number>(
     scope: CoroutineScope,
-    hidden: Val<Boolean>,
-    disabled: Val<Boolean>,
-    tooltip: String?,
+    visible: Val<Boolean>,
+    enabled: Val<Boolean>,
+    tooltip: Val<String?>,
     label: String?,
     labelVal: Val<String>?,
     preferredLabelPosition: LabelPosition,
@@ -19,8 +19,8 @@ abstract class NumberInput<T : Number>(
     step: Int?,
 ) : Input<T>(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,

webui/src/main/kotlin/world/phantasmal/webui/widgets/Select.kt

@@ -4,18 +4,15 @@ import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import org.w3c.dom.events.KeyboardEvent
 import org.w3c.dom.events.MouseEvent
-import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
-import world.phantasmal.observable.value.mutableVal
-import world.phantasmal.observable.value.value
+import world.phantasmal.observable.value.*
 import world.phantasmal.webui.dom.Icon
 import world.phantasmal.webui.dom.div
 
 class Select<T : Any>(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     label: String? = null,
     labelVal: Val<String>? = null,
     preferredLabelPosition: LabelPosition = LabelPosition.Before,
@@ -27,8 +24,8 @@ class Select<T : Any>(
     private val onSelect: (T) -> Unit = {},
 ) : LabelledControl(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,
@@ -38,7 +35,7 @@ class Select<T : Any>(
     private val selected: Val<T?> = selectedVal ?: value(selected)
 
     private val buttonText = mutableVal(" ")
-    private val menuHidden = mutableVal(true)
+    private val menuVisible = mutableVal(false)
 
     private lateinit var menu: Menu<T>
     private var justOpened = false
@@ -52,7 +49,7 @@ class Select<T : Any>(
 
             addWidget(Button(
                 scope,
-                disabled = disabled,
+                enabled = enabled,
                 textVal = buttonText,
                 iconRight = Icon.TriangleDown,
                 onMouseDown = ::onButtonMouseDown,
@@ -61,19 +58,19 @@ class Select<T : Any>(
             ))
             menu = addWidget(Menu(
                 scope,
-                hidden = menuHidden,
-                disabled = disabled,
+                visible = menuVisible,
+                enabled = enabled,
                 itemsVal = items,
                 itemToString = itemToString,
                 onSelect = ::select,
-                onCancel = { menuHidden.value = true },
+                onCancel = { menuVisible.value = false },
             ))
         }
 
     private fun onButtonMouseDown(e: MouseEvent) {
         e.stopPropagation()
-        justOpened = menuHidden.value
-        menuHidden.value = false
+        justOpened = !menuVisible.value
+        menuVisible.value = true
         selected.value?.let(menu::highlightItem)
     }
 
@@ -81,7 +78,7 @@ class Select<T : Any>(
         if (justOpened) {
             menu.focus()
         } else {
-            menuHidden.value = true
+            menuVisible.value = false
         }
 
         justOpened = false
@@ -93,8 +90,8 @@ class Select<T : Any>(
                 e.preventDefault()
                 e.stopPropagation()
 
-                justOpened = menuHidden.value
-                menuHidden.value = false
+                justOpened = !menuVisible.value
+                menuVisible.value = true
                 selected.value?.let(menu::highlightItem)
                 menu.focus()
             }
@@ -134,7 +131,7 @@ class Select<T : Any>(
     }
 
     private fun select(item: T) {
-        menuHidden.value = true
+        menuVisible.value = false
         buttonText.value = itemToString(item)
         onSelect(item)
     }

webui/src/main/kotlin/world/phantasmal/webui/widgets/TabContainer.kt

@@ -3,7 +3,8 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.eq
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.controllers.Tab
 import world.phantasmal.webui.controllers.TabController
 import world.phantasmal.webui.dom.div
@@ -11,11 +12,11 @@ import world.phantasmal.webui.dom.span
 
 class TabContainer<T : Tab>(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
     private val ctrl: TabController<T>,
     private val createWidget: (CoroutineScope, T) -> Widget,
-) : Widget(scope, hidden, disabled) {
+) : Widget(scope, visible, enabled) {
     override fun Node.createElement() =
         div {
             className = "pw-tab-container"
@@ -48,7 +49,7 @@ class TabContainer<T : Tab>(
                     addChild(
                         LazyLoader(
                             scope,
-                            hidden = ctrl.activeTab.map { it != tab },
+                            visible = ctrl.activeTab eq tab,
                             createWidget = { scope -> createWidget(scope, tab) }
                         )
                     )
@@ -57,7 +58,7 @@ class TabContainer<T : Tab>(
         }
 
     init {
-        observe(selfOrAncestorHidden, ctrl::hiddenChanged)
+        observe(selfOrAncestorVisible, ctrl::visibleChanged)
     }
 
     companion object {

webui/src/main/kotlin/world/phantasmal/webui/widgets/TextArea.kt

@@ -3,15 +3,16 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.dom.div
 import world.phantasmal.webui.dom.textarea
 
 class TextArea(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     label: String? = null,
     labelVal: Val<String>? = null,
     preferredLabelPosition: LabelPosition = LabelPosition.Before,
@@ -24,8 +25,8 @@ class TextArea(
     private val cols: Int? = null,
 ) : LabelledControl(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,
@@ -38,7 +39,7 @@ class TextArea(
             textarea {
                 className = "pw-text-area-inner"
 
-                observe(this@TextArea.disabled) { disabled = it }
+                observe(this@TextArea.enabled) { disabled = !it }
 
                 if (setValue != null) {
                     onchange = { setValue.invoke(value) }

webui/src/main/kotlin/world/phantasmal/webui/widgets/TextInput.kt

@@ -3,13 +3,14 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.HTMLInputElement
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.nullVal
+import world.phantasmal.observable.value.trueVal
 
 class TextInput(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
-    tooltip: String? = null,
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
+    tooltip: Val<String?> = nullVal(),
     label: String? = null,
     labelVal: Val<String>? = null,
     preferredLabelPosition: LabelPosition = LabelPosition.Before,
@@ -19,8 +20,8 @@ class TextInput(
     maxLength: Int? = null,
 ) : Input<String>(
     scope,
-    hidden,
-    disabled,
+    visible,
+    enabled,
     tooltip,
     label,
     labelVal,

webui/src/main/kotlin/world/phantasmal/webui/widgets/Toolbar.kt

@@ -3,15 +3,15 @@ package world.phantasmal.webui.widgets
 import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.Node
 import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.trueVal
 import world.phantasmal.webui.dom.div
 
 class Toolbar(
     scope: CoroutineScope,
-    hidden: Val<Boolean> = falseVal(),
-    disabled: Val<Boolean> = falseVal(),
+    visible: Val<Boolean> = trueVal(),
+    enabled: Val<Boolean> = trueVal(),
     children: List<Widget>,
-) : Widget(scope, hidden, disabled) {
+) : Widget(scope, visible, enabled) {
     private val childWidgets = children
 
     override fun Node.createElement() =

webui/src/main/kotlin/world/phantasmal/webui/widgets/Widget.kt

@@ -5,12 +5,9 @@ import kotlinx.coroutines.CoroutineScope
 import org.w3c.dom.*
 import world.phantasmal.core.disposable.disposable
 import world.phantasmal.observable.Observable
-import world.phantasmal.observable.value.Val
-import world.phantasmal.observable.value.falseVal
+import world.phantasmal.observable.value.*
 import world.phantasmal.observable.value.list.ListVal
 import world.phantasmal.observable.value.list.ListValChangeEvent
-import world.phantasmal.observable.value.mutableVal
-import world.phantasmal.observable.value.or
 import world.phantasmal.webui.DisposableContainer
 
 abstract class Widget(
@@ -18,15 +15,15 @@ abstract class Widget(
     /**
      * By default determines the hidden attribute of its [element].
      */
-    val hidden: Val<Boolean> = falseVal(),
+    val visible: Val<Boolean> = trueVal(),
     /**
      * By default determines the disabled attribute of its [element] and whether or not the
      * "pw-disabled" class is added.
      */
-    val disabled: Val<Boolean> = falseVal(),
-    val tooltip: String? = null,
+    val enabled: Val<Boolean> = trueVal(),
+    val tooltip: Val<String?> = nullVal(),
 ) : DisposableContainer() {
-    private val _ancestorHidden = mutableVal(false)
+    private val _ancestorVisible = mutableVal(true)
     private val _children = mutableListOf<Widget>()
     private var initResizeObserverRequested = false
     private var resizeObserverInitialized = false
@@ -34,22 +31,28 @@ abstract class Widget(
     private val elementDelegate = lazy {
         val el = document.createDocumentFragment().createElement()
 
-        observe(hidden) { hidden ->
-            el.hidden = hidden
-            children.forEach { setAncestorHidden(it, hidden || ancestorHidden.value) }
+        observe(visible) { visible ->
+            el.hidden = !visible
+            children.forEach { setAncestorVisible(it, visible && ancestorVisible.value) }
         }
 
-        observe(disabled) { disabled ->
-            if (disabled) {
-                el.setAttribute("disabled", "")
-                el.classList.add("pw-disabled")
-            } else {
+        observe(enabled) { enabled ->
+            if (enabled) {
                 el.removeAttribute("disabled")
                 el.classList.remove("pw-disabled")
+            } else {
+                el.setAttribute("disabled", "")
+                el.classList.add("pw-disabled")
             }
         }
 
-        tooltip?.let { el.title = it }
+        observe(tooltip) { tooltip ->
+            if (tooltip == null) {
+                el.removeAttribute("title")
+            } else {
+                el.title = tooltip
+            }
+        }
 
         if (initResizeObserverRequested) {
             initResizeObserver(el)
@@ -65,14 +68,14 @@ abstract class Widget(
     val element: HTMLElement by elementDelegate
 
     /**
-     * True if any of this widget's ancestors are [hidden], false otherwise.
+     * True if this widget's ancestors are [visible], false otherwise.
      */
-    val ancestorHidden: Val<Boolean> = _ancestorHidden
+    val ancestorVisible: Val<Boolean> = _ancestorVisible
 
     /**
-     * True if this widget or any of its ancestors are [hidden], false otherwise.
+     * True if this widget and all of its ancestors are [visible], false otherwise.
      */
-    val selfOrAncestorHidden: Val<Boolean> = hidden or ancestorHidden
+    val selfOrAncestorVisible: Val<Boolean> = visible and ancestorVisible
 
     val children: List<Widget> = _children
 
@@ -122,7 +125,7 @@ abstract class Widget(
     protected fun <T : Widget> Node.addChild(child: T): T {
         addDisposable(child)
         _children.add(child)
-        setAncestorHidden(child, selfOrAncestorHidden.value)
+        setAncestorVisible(child, selfOrAncestorVisible.value)
         appendChild(child.element)
         return child
     }
@@ -239,13 +242,13 @@ abstract class Widget(
             STYLE_EL.append(style)
         }
 
-        protected fun setAncestorHidden(widget: Widget, hidden: Boolean) {
-            widget._ancestorHidden.value = hidden
+        protected fun setAncestorVisible(widget: Widget, visible: Boolean) {
+            widget._ancestorVisible.value = visible
 
-            if (widget.hidden.value) return
+            if (!widget.visible.value) return
 
             widget.children.forEach {
-                setAncestorHidden(it, widget.selfOrAncestorHidden.value)
+                setAncestorVisible(it, widget.selfOrAncestorVisible.value)
             }
         }
     }

webui/src/test/kotlin/world/phantasmal/webui/widgets/WidgetTests.kt

@@ -14,59 +14,59 @@ import kotlin.test.assertTrue
 
 class WidgetTests : WebuiTestSuite() {
     @Test
-    fun ancestorHidden_and_selfOrAncestorHidden_should_update_when_hidden_changes() = test {
-        val parentHidden = mutableVal(false)
-        val childHidden = mutableVal(false)
+    fun ancestorVisible_and_selfOrAncestorVisible_should_update_when_visible_changes() = test {
+        val parentVisible = mutableVal(true)
+        val childVisible = mutableVal(true)
         val grandChild = DummyWidget(scope)
-        val child = DummyWidget(scope, childHidden, grandChild)
-        val parent = disposer.add(DummyWidget(scope, parentHidden, child))
+        val child = DummyWidget(scope, childVisible, grandChild)
+        val parent = disposer.add(DummyWidget(scope, parentVisible, child))
 
         parent.element // Ensure widgets are fully initialized.
 
-        assertFalse(parent.ancestorHidden.value)
-        assertFalse(parent.selfOrAncestorHidden.value)
-        assertFalse(child.ancestorHidden.value)
-        assertFalse(child.selfOrAncestorHidden.value)
-        assertFalse(grandChild.ancestorHidden.value)
-        assertFalse(grandChild.selfOrAncestorHidden.value)
+        assertTrue(parent.ancestorVisible.value)
+        assertTrue(parent.selfOrAncestorVisible.value)
+        assertTrue(child.ancestorVisible.value)
+        assertTrue(child.selfOrAncestorVisible.value)
+        assertTrue(grandChild.ancestorVisible.value)
+        assertTrue(grandChild.selfOrAncestorVisible.value)
 
-        parentHidden.value = true
+        parentVisible.value = false
 
-        assertFalse(parent.ancestorHidden.value)
-        assertTrue(parent.selfOrAncestorHidden.value)
-        assertTrue(child.ancestorHidden.value)
-        assertTrue(child.selfOrAncestorHidden.value)
-        assertTrue(grandChild.ancestorHidden.value)
-        assertTrue(grandChild.selfOrAncestorHidden.value)
+        assertTrue(parent.ancestorVisible.value)
+        assertFalse(parent.selfOrAncestorVisible.value)
+        assertFalse(child.ancestorVisible.value)
+        assertFalse(child.selfOrAncestorVisible.value)
+        assertFalse(grandChild.ancestorVisible.value)
+        assertFalse(grandChild.selfOrAncestorVisible.value)
 
-        childHidden.value = true
-        parentHidden.value = false
+        childVisible.value = false
+        parentVisible.value = true
 
-        assertFalse(parent.ancestorHidden.value)
-        assertFalse(parent.selfOrAncestorHidden.value)
-        assertFalse(child.ancestorHidden.value)
-        assertTrue(child.selfOrAncestorHidden.value)
-        assertTrue(grandChild.ancestorHidden.value)
-        assertTrue(grandChild.selfOrAncestorHidden.value)
+        assertTrue(parent.ancestorVisible.value)
+        assertTrue(parent.selfOrAncestorVisible.value)
+        assertTrue(child.ancestorVisible.value)
+        assertFalse(child.selfOrAncestorVisible.value)
+        assertFalse(grandChild.ancestorVisible.value)
+        assertFalse(grandChild.selfOrAncestorVisible.value)
     }
 
     @Test
-    fun added_child_widgets_should_have_ancestorHidden_and_selfOrAncestorHidden_set_correctly() =
+    fun added_child_widgets_should_have_ancestorVisible_and_selfOrAncestorVisible_set_correctly() =
         test {
-            val parent = disposer.add(DummyWidget(scope, hidden = trueVal()))
+            val parent = disposer.add(DummyWidget(scope, visible = falseVal()))
             val child = parent.addChild(DummyWidget(scope))
 
-            assertFalse(parent.ancestorHidden.value)
-            assertTrue(parent.selfOrAncestorHidden.value)
-            assertTrue(child.ancestorHidden.value)
-            assertTrue(child.selfOrAncestorHidden.value)
+            assertTrue(parent.ancestorVisible.value)
+            assertFalse(parent.selfOrAncestorVisible.value)
+            assertFalse(child.ancestorVisible.value)
+            assertFalse(child.selfOrAncestorVisible.value)
         }
 
     private inner class DummyWidget(
         scope: CoroutineScope,
-        hidden: Val<Boolean> = falseVal(),
+        visible: Val<Boolean> = trueVal(),
         private val child: Widget? = null,
-    ) : Widget(scope, hidden = hidden) {
+    ) : Widget(scope, visible = visible) {
         override fun Node.createElement() = div {
             child?.let { addChild(it) }
         }