A game that is more than one scene needs a way from one to the next, so the graph can load one (#39)

Load scene sets a flag on the World like Stop and Restart do; the swap
happens after the tick, and the last wish of a tick wins. Exactly one
thing crosses over: the SharedState, so global variables and global
machines keep what they know while everything local is built anew.
Loading the scene that is already running rebuilds it, which is what
"reset the level" means when the score must survive.

A run starts either from the start scene (a new project field, set from
the asset panel and marked with an arrow in the list) or from whatever
scene is open. The toggle next to Play says which question is asked and
remembers the answer in localStorage: it is a way of working, not part
of the game.

Sounds no longer end because a scene ends. A voice belongs to the
behavior that started it and stops right after On destroy, so the scene
swap is no longer a special case at all. For the enemy that plays its
explosion and destroys itself in the same breath there is now a
detached input on Play sound.

The scene selector stays usable while a run plays, because a double
click in the asset panel already did the same thing unguarded. It moves
what the editor looks at, not what the run plays.

src/core/commands/assetCommands.ts

@@ -344,6 +344,31 @@ export class ResetAssetPropertyCommand extends AssetPropertyCommand {
   }
 }
 
+/**
+ * Names the scene a real run begins with, or takes that name back (`''`). A deleted scene is not
+ * cleaned up here on purpose: the id then simply reads as none, and an undo of the deletion brings
+ * the start scene back with it.
+ */
+export class SetStartSceneCommand implements Command {
+  readonly label = 'Set start scene';
+  private previous = '';
+
+  constructor(private readonly sceneId: string) {}
+
+  execute(project: Project): void {
+    if (this.sceneId) {
+      const asset = requireAsset(project, this.sceneId);
+      if (asset.type !== 'scene') throw new CommandError(`${asset.name} is not a scene`);
+    }
+    this.previous = project.startSceneId;
+    project.startSceneId = this.sceneId;
+  }
+
+  undo(project: Project): void {
+    project.startSceneId = this.previous;
+  }
+}
+
 export class SetActiveSceneCommand implements Command {
   readonly label = 'Switch scene';
   private previous = '';

src/core/graph/NodeContext.ts

@@ -125,10 +125,20 @@ export interface NodeContext {
    * because the asset says what a sound *is* and the node only colours it. Every call is a voice of
    * its own: two plays overlap, whether they loop or not, which is what makes one sound effect
    * fired twice in a row audible twice.
+   *
+   * The voice belongs to the behavior that started it and ends when that behavior is destroyed –
+   * a sound is something a thing in the world is doing. `detached` gives that up on purpose, for
+   * the one case where the rule is in the way: the death sound of an object that destroys itself.
    */
-  playSound(assetId: string, volume: number, loop: boolean | undefined): void;
+  playSound(assetId: string, volume: number, loop: boolean | undefined, detached: boolean): void;
   /** Silences every voice of that sound; `''` silences everything this run started. */
   stopSound(assetId: string): void;
+  /**
+   * Asks for another scene, by asset id. The swap happens once the frame is over – a graph must be
+   * able to finish the flow it is in, and half a frame in the old world and half in the new one is
+   * the kind of thing nobody can debug. The last request of a frame is the one that happens.
+   */
+  loadScene(sceneId: string): void;
   /**
    * The position is local to `parent` (world when none); null when the asset does not exist or
    * cannot be placed. A spawned behavior runs its `On start` on the next tick.

src/core/graph/nodes/action/LoadScene.ts

@@ -0,0 +1,35 @@
+import type { NodeDef, PortDef } from '../../NodeRegistry';
+import { asString } from '../../values';
+import { exec } from '../shared';
+
+/**
+ * The `scene` input; unconnected it shows the scene picker, the way `soundPort` shows the sound
+ * one. A `Scene` literal is what feeds it through an edge.
+ */
+const scenePort = (): PortDef => ({
+  key: 'scene',
+  label: 'scene',
+  type: 'string',
+  field: { key: 'scene', label: 'Scene', kind: 'scene', default: '' },
+});
+
+export const LoadScene: NodeDef = {
+  type: 'LoadScene',
+  label: 'Load scene',
+  category: 'action',
+  description:
+    'Replaces the running scene with another one. Everything of the old scene is gone – its ' +
+    'objects, their behaviors and the sounds those behaviors were playing; what a Variables asset ' +
+    'holds and where a global state machine stands carry over, because those belong to the run and ' +
+    'not to the scene. Naming the scene that is already running rebuilds it, which is how a level ' +
+    'is reset without losing the score. The swap happens once the frame has finished, so the rest ' +
+    'of this graph still runs; asked twice in one frame, the last answer wins.',
+  inputs: [exec(), scenePort()],
+  outputs: [exec()],
+  params: [],
+  run: (ctx, inputs) => {
+    const scene = asString(inputs.scene);
+    if (scene) ctx.loadScene(scene);
+    return { next: 'exec' };
+  },
+};

src/core/graph/nodes/action/PlaySound.ts

@@ -1,5 +1,5 @@
 import type { NodeDef, PortDef } from '../../NodeRegistry';
-import { asNumber, asString, type Value } from '../../values';
+import { asBoolean, asNumber, asString, type Value } from '../../values';
 import { exec, num, soundPort, withDefault } from '../shared';
 
 /**
@@ -34,6 +34,19 @@ export function loopChoice(value: Value | undefined): boolean | undefined {
   return undefined;
 }
 
+/**
+ * The way out of the ownership rule, off by default. Needed for one shape that is common enough to
+ * deserve it: an object that plays its own death sound and destroys itself in the same flow – the
+ * voice would be cut off in the instant it started, because the behavior holding it is gone.
+ */
+const detachedPort = (): PortDef => ({
+  key: 'detached',
+  label: 'detached',
+  type: 'boolean',
+  default: false,
+  field: { key: 'detached', label: 'Detached (outlives this object)', kind: 'boolean', default: false },
+});
+
 export const PlaySound: NodeDef = {
   type: 'PlaySound',
   label: 'Play sound',
@@ -42,13 +55,15 @@ export const PlaySound: NodeDef = {
     'Plays a sound asset. "volume" multiplies the volume the asset itself carries and "loop" either ' +
     "leaves the asset's own setting alone or overrules it, so the asset says what the sound is and " +
     'the node only colours it. Each play is a voice of its own: firing this twice in a row is heard ' +
-    'twice, overlapping.',
-  inputs: [exec(), soundPort(), withDefault(num('volume'), 1), loopPort()],
+    'twice, overlapping. The sound belongs to this object and stops when the object is destroyed, ' +
+    'unless "detached" is on.',
+  inputs: [exec(), soundPort(), withDefault(num('volume'), 1), loopPort(), detachedPort()],
   outputs: [exec()],
   params: [],
   run: (ctx, inputs) => {
     const sound = asString(inputs.sound);
-    if (sound) ctx.playSound(sound, asNumber(inputs.volume), loopChoice(inputs.loop));
+    if (sound)
+      ctx.playSound(sound, asNumber(inputs.volume), loopChoice(inputs.loop), asBoolean(inputs.detached));
     return { next: 'exec' };
   },
 };

src/core/graph/nodes/data/literals.ts

@@ -40,6 +40,11 @@ export const SoundLiteral = literal(
   'sound',
   'A constant sound, for a "Play sound" or a "Stop sound" to work on.',
 );
+export const SceneLiteral = literal(
+  'Scene',
+  'scene',
+  'A constant scene, for a "Load scene" to work on.',
+);
 export const ListLiteral = literal(
   'List',
   'list',

src/core/graph/nodes/index.ts

@@ -1,6 +1,7 @@
 import { type NodeDef, registerNode } from '../NodeRegistry';
 import { Destroy } from './action/Destroy';
 import { Emit } from './action/Emit';
+import { LoadScene } from './action/LoadScene';
 import { Log } from './action/Log';
 import { Move } from './action/Move';
 import { PlaySound } from './action/PlaySound';
@@ -34,6 +35,7 @@ import {
   BooleanLiteral,
   ListLiteral,
   NumberLiteral,
+  SceneLiteral,
   SoundLiteral,
   StateMachineLiteral,
   StringLiteral,
@@ -150,6 +152,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   TextureLiteral,
   StateMachineLiteral,
   SoundLiteral,
+  SceneLiteral,
   Add,
   Subtract,
   Multiply,
@@ -201,6 +204,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Emit,
   PlaySound,
   StopSound,
+  LoadScene,
   Log,
   StopGame,
   RestartGame,

src/core/model/Project.ts

@@ -17,10 +17,28 @@ export interface Project {
   assets: Asset[];
   /** Empty string = none. */
   activeSceneId: string;
+  /**
+   * Where the game begins – what a player would see first, as opposed to whatever scene happens to
+   * be open in the editor. Empty (or dangling) means none has been named, and a run falls back to
+   * the active scene; no format bump needed for that, since an older project reads as exactly that.
+   */
+  startSceneId: string;
 }
 
 export function createEmptyProject(id: string, name: string): Project {
-  return { formatVersion: PROJECT_FORMAT_VERSION, id, name, assets: [], activeSceneId: '' };
+  return {
+    formatVersion: PROJECT_FORMAT_VERSION,
+    id,
+    name,
+    assets: [],
+    activeSceneId: '',
+    startSceneId: '',
+  };
+}
+
+/** The scene a real run starts from: the named one, or the active one while none is named. */
+export function startScene(project: Project): SceneAsset | undefined {
+  return findScene(project, project.startSceneId) ?? findScene(project, project.activeSceneId);
 }
 
 export function findAsset(project: Project, assetId: string): Asset | undefined {

src/core/properties/kinds.ts

@@ -120,6 +120,20 @@ export const KINDS = [
     field: true,
     port: true,
   },
+  /** A scene asset's id – a text, so it is tagged in a list like a texture. */
+  {
+    kind: 'scene',
+    label: 'Scene',
+    mark: '▣',
+    travels: 'string',
+    empty: '',
+    coerce: text,
+    tagged: true,
+    reference: true,
+    variable: true,
+    field: true,
+    port: true,
+  },
   { kind: 'object', label: 'Object', mark: '◇', travels: 'object', port: true },
   { kind: 'list', label: 'List', mark: '≡', travels: 'list', variable: true, port: true },
   { kind: 'struct', label: 'Struct', mark: '⊞', travels: 'struct', variable: true, port: true },

src/core/store/serialize.ts

@@ -178,12 +178,20 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
     activeSceneId = first ? first.id : '';
   }
 
+  // A dangling start scene reads as none at all – the run then falls back to the active scene,
+  // which is also what every project written before there was such a field does.
+  const startSceneId =
+    typeof r.startSceneId === 'string' && assets.some((a) => a.type === 'scene' && a.id === r.startSceneId)
+      ? r.startSceneId
+      : '';
+
   const project: Project = {
     formatVersion: PROJECT_FORMAT_VERSION,
     id: typeof r.id === 'string' && r.id ? r.id : fallbackId,
     name: typeof r.name === 'string' ? r.name : 'Untitled',
     assets,
     activeSceneId,
+    startSceneId,
   };
   return project;
 }

src/editor/Editor.ts

@@ -39,6 +39,7 @@ import { ParticlesEditor } from './particles/ParticlesEditor';
 import { ShaderEditor } from './shader/ShaderEditor';
 import { StateMachineEditor } from './statemachine/StateMachineEditor';
 import { installShortcuts } from './shortcuts';
+import type { PlayFrom } from './settings';
 import type { EditorState } from './state';
 import { TextureEditor } from './texture/TextureEditor';
 import { installFileImport } from './fileImport';
@@ -399,8 +400,31 @@ export class Editor implements PlayHost {
     return this.editorRun.debugInfo();
   }
 
-  /** Play state for the check scripts; `time` is the world clock a pause has to freeze. */
-  playInfo(): { playing: boolean; paused: boolean; time: number } {
-    return { playing: this.play.isPlaying, paused: this.play.isPaused, time: this.play.time };
+  /** What the start-scene switch says, and the switch itself – for the check scripts. */
+  playFromInfo(): { mode: PlayFrom; label: string; scene: string } {
+    return this.toolbar.playFromInfo();
+  }
+
+  get playFromButton(): Widget {
+    return this.toolbar.playFromButton;
+  }
+
+  /** The scene selector, so a check can reach it – notably while a run is on (test hook). */
+  get sceneButton(): Widget {
+    return this.toolbar.sceneButton;
+  }
+
+  /**
+   * Play state for the check scripts; `time` is the world clock a pause has to freeze. `sceneId`
+   * is the scene the run is *in* – the only way a check can see a `Load scene` happen, since the
+   * swap leaves the editor's own active scene alone.
+   */
+  playInfo(): { playing: boolean; paused: boolean; time: number; sceneId: string } {
+    return {
+      playing: this.play.isPlaying,
+      paused: this.play.isPaused,
+      time: this.play.time,
+      sceneId: this.play.sceneId,
+    };
   }
 }

src/editor/EditorRun.ts

@@ -131,6 +131,13 @@ export class EditorRun {
 
   /** True when the run ended: a behavior failed, stopped the game, or asked to start over. */
   private control(world: World): boolean {
+    if (world.loadSceneRequested) {
+      // The editor run is the scene you are editing, brought to life – swapping it for another one
+      // would mean editing a scene you are not looking at. Said once per ask, not once per frame.
+      world.log.push(`Load scene is ignored in the editor run (${world.loadSceneRequested})`);
+      console.warn('[editor run] Load scene only works while playing');
+      world.loadSceneRequested = '';
+    }
     if (world.restartRequested) {
       this.stop();
       this.stale = true;

src/editor/PlaySession.ts

@@ -93,6 +93,11 @@ export class PlaySession {
     return this.world?.time ?? 0;
   }
 
+  /** The scene the run is in – not the editor's active one, which a `Load scene` leaves alone. */
+  get sceneId(): string {
+    return this.world?.sceneId ?? '';
+  }
+
   syncFromModel(): void {
     this.world?.syncFromModel();
     this.behaviors?.syncDeclarations();
@@ -104,7 +109,7 @@ export class PlaySession {
   }
 
   play(): void {
-    const scene = this.state.activeScene;
+    const scene = this.state.sceneToPlay();
     if (!scene) return;
     ui().focus.blur();
     // A run starts at second zero, whatever the editor's previews have been counting up to.
@@ -120,6 +125,26 @@ export class PlaySession {
     if (world.stopRequested) this.stop();
   }
 
+  /**
+   * Swaps the running scene for another one, inside the same run: the world is torn down and built
+   * again, while what belongs to the run – the clock, the shared variables, the global machines –
+   * carries on. The sounds of the old scene end with it, because the behaviors playing them are
+   * destroyed; a voice that was started detached is nobody's and plays on.
+   */
+  private loadScene(sceneId: string): void {
+    const old = this.world;
+    const runtime = this.state.runtimeProject;
+    if (!old || !runtime) return;
+    const carried = this.behaviors?.shared ?? null;
+    old.destroy();
+    const world = World.instantiate(runtime, sceneId);
+    this.world = world;
+    this.behaviors = addGameSystems(world, runtime, this.input, undefined, soundLibrary, carried);
+    this.host.gameView.setWorld(world);
+    this.state.setNotice(this.playNotice(), 'quiet');
+    if (world.stopRequested) this.stop();
+  }
+
   private playNotice(): string {
     if (this.state.paused) return 'Paused · Ctrl+Enter resumes · Esc stops';
     return this.world?.activeCamera()
@@ -168,6 +193,10 @@ export class PlaySession {
       this.play();
       return;
     }
+    if (world.loadSceneRequested) {
+      this.loadScene(world.loadSceneRequested);
+      return;
+    }
     // Mirror what the systems did into the runtime copy so the panels show live values.
     if (world.syncToModel().structure) this.state.preview();
     this.state.runtimeTick();

src/editor/debugHook.ts

@@ -423,6 +423,11 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     },
     world: () => editor.worldSnapshot(),
     playInfo: () => editor.playInfo(),
+    /** The start-scene switch: what it says, and where to click it (its caption is elided). */
+    playFrom: () => editor.playFromInfo(),
+    playFromAt: () => centreOf(editor.playFromButton),
+    /** The scene selector: where it is, and whether it takes a click (it does, play mode or not). */
+    sceneSelect: () => ({ ...centreOf(editor.sceneButton), enabled: editor.sceneButton.enabled }),
     /** Behaviors running without Play (`Run in editor`), and how long they have been at it. */
     editorRun: () => editor.editorRunInfo(),
     /** Rows the hierarchy shows – collapsed subtrees are not in here. */

src/editor/inspector/fieldFactory.ts

@@ -242,6 +242,11 @@ const REFERENCES: Record<ReferenceKind, Reference> = {
     label: assetRefLabel,
     choices: (project) => assetRefChoices(project, ['sound']).map((a) => ({ id: a.id, label: a.name })),
   },
+  scene: {
+    none: NO_ASSET_LABEL,
+    label: assetRefLabel,
+    choices: (project) => assetRefChoices(project, ['scene']).map((a) => ({ id: a.id, label: a.name })),
+  },
 };
 
 /** An id in the words of its reference kind; nothing for a kind that names nothing. */
@@ -368,6 +373,7 @@ export function createField(
     case 'texture':
     case 'statemachine':
     case 'sound':
+    case 'scene':
       return referenceField(ctx, def, REFERENCES[def.kind], value, handlers);
   }
 }

src/editor/panels/AssetPanel.ts

@@ -1,5 +1,9 @@
 import { AddAnimationFramesCommand } from '../../core/commands/animationCommands';
-import { CreateAssetCommand, SetActiveSceneCommand } from '../../core/commands/assetCommands';
+import {
+  CreateAssetCommand,
+  SetActiveSceneCommand,
+  SetStartSceneCommand,
+} from '../../core/commands/assetCommands';
 import { oneStep } from '../../core/commands/CompoundCommand';
 import { nextAssetName } from '../../core/commands/naming';
 import {
@@ -85,13 +89,23 @@ export class AssetPanel extends Panel {
     );
   }
 
+  /**
+   * What a scene row says about itself: `●` the one the editor shows, `▶` the one a run begins
+   * with. Two marks rather than one, because they are two different questions and often two
+   * different scenes – the level you are editing versus the title screen.
+   */
+  private sceneHint(id: string): string {
+    const { activeSceneId, startSceneId } = this.state.project;
+    const marks = `${id === startSceneId ? '▶' : ''}${id === activeSceneId ? '●' : ''}`;
+    return marks ? `scene ${marks}` : 'scene';
+  }
+
   private refresh(): void {
-    const active = this.state.project.activeSceneId;
     this.list.setItems(
       this.state.project.assets.map((asset) => ({
         id: asset.id,
         label: asset.name,
-        hint: asset.type === 'scene' && asset.id === active ? 'scene ●' : asset.type,
+        hint: asset.type === 'scene' ? this.sceneHint(asset.id) : asset.type,
         swatch: hasColorSwatch(asset.type) ? Number(asset.props.color ?? 0) : undefined,
         overridden: assetDeviates(this.state.project, asset),
       })),
@@ -187,6 +201,11 @@ export class AssetPanel extends Panel {
         onSelect: () => this.state.execute(new SetActiveSceneCommand(id)),
         disabled: this.state.project.activeSceneId === id,
       });
+      const isStart = this.state.project.startSceneId === id;
+      items.push({
+        label: isStart ? 'Clear start scene' : 'Set as start scene',
+        onSelect: () => this.state.execute(new SetStartSceneCommand(isStart ? '' : id)),
+      });
     }
     if (asset.type === 'texture') {
       items.push({

src/editor/panels/Toolbar.ts

@@ -1,7 +1,8 @@
 import { Graphics } from 'pixi.js';
 import { RenameProjectCommand } from '../../core/commands/projectCommands';
 import { listScenes } from '../../core/model/Project';
-import { Button, Dropdown, HStack, Label, Spacer, TextField, Theme, type Widget } from '../../ui';
+import { Button, Dropdown, HStack, Label, Menu, Spacer, TextField, Theme, type Widget } from '../../ui';
+import type { PlayFrom } from '../settings';
 import type { EditorState, SaveStatus } from '../state';
 
 /** What the toolbar buttons trigger; the editor supplies the implementations. */
@@ -27,9 +28,10 @@ const SAVE_LABELS: Record<SaveStatus, string> = {
 export class Toolbar extends HStack {
   private readonly bg = new Graphics();
   private readonly projectName: TextField;
-  private readonly sceneButton: Dropdown;
+  private readonly sceneSelect: Dropdown;
   private readonly undoButton: Button;
   private readonly redoButton: Button;
+  private readonly playFrom: Dropdown;
   private readonly playButton: Button;
   private readonly pauseButton: Button;
   private readonly status: Label;
@@ -58,11 +60,11 @@ export class Toolbar extends HStack {
     this.add(new Spacer(Theme.padding));
 
     this.add(new Label('Scene', { style: 'muted' }));
-    this.sceneButton = new Dropdown('—', {
+    this.sceneSelect = new Dropdown('—', {
       width: 160,
-      onClick: () => actions.openSceneMenu(this.sceneButton),
+      onClick: () => actions.openSceneMenu(this.sceneSelect),
     });
-    this.add(this.sceneButton);
+    this.add(this.sceneSelect);
     this.add(new Spacer(Theme.padding));
 
     this.undoButton = new Button('Undo', { onClick: () => state.undo() });
@@ -71,6 +73,12 @@ export class Toolbar extends HStack {
     this.add(this.redoButton);
 
     this.add(new Spacer(), 1);
+    // Next to Play, because it answers a question about Play: which scene does this start?
+    this.playFrom = new Dropdown('—', {
+      width: 130,
+      onClick: () => this.openPlayFromMenu(),
+    });
+    this.add(this.playFrom);
     this.playButton = new Button('▶  Play', {
       variant: 'primary',
       width: 110,
@@ -102,6 +110,33 @@ export class Toolbar extends HStack {
     this.refresh();
   }
 
+  /** The start-scene switch for the check scripts: what it says, and where to click it. */
+  playFromInfo(): { mode: PlayFrom; label: string; scene: string } {
+    return {
+      mode: this.state.playFrom,
+      label: this.playFrom.text,
+      scene: this.state.sceneToPlay()?.name ?? '',
+    };
+  }
+
+  /** The switch itself, so a check can open it without hunting for its (elided) caption. */
+  get playFromButton(): Dropdown {
+    return this.playFrom;
+  }
+
+  /** The scene selector, for the check scripts – its caption is a scene name, not a fixed label. */
+  get sceneButton(): Dropdown {
+    return this.sceneSelect;
+  }
+
+  private openPlayFromMenu(): void {
+    const mark = (from: PlayFrom, text: string) => (this.state.playFrom === from ? `● ${text}` : text);
+    new Menu([
+      { label: mark('start', 'Start scene'), onSelect: () => this.state.setPlayFrom('start') },
+      { label: mark('active', 'Active scene'), onSelect: () => this.state.setPlayFrom('active') },
+    ]).openBelow(this.playFrom);
+  }
+
   /** Frame rate readout; `null` while no measurement is available yet. */
   setFps(fps: number | null): void {
     this.fps.setText(fps === null ? '– fps' : `${fps} fps`);
@@ -112,20 +147,28 @@ export class Toolbar extends HStack {
     this.projectName.setValue(state.project.name);
     const scene = state.activeScene;
     const scenes = listScenes(state.project);
-    this.sceneButton.setText(scene ? scene.name : scenes.length ? 'Select scene' : 'No scene');
+    this.sceneSelect.setText(scene ? scene.name : scenes.length ? 'Select scene' : 'No scene');
     const editing = state.mode === 'edit';
+    const from = state.sceneToPlay();
+    // The mode, worded exactly like the menu entry it came from – a caption that reads differently
+    // from the thing you picked makes you check twice whether you picked the right one.
+    this.playFrom.setText(state.playFrom === 'active' ? 'Active scene' : 'Start scene');
+    this.playFrom.setEnabled(editing);
     // Undo/redo work in play mode too – on the runtime copy's own history.
     this.undoButton.setEnabled(state.commands.canUndo);
     this.redoButton.setEnabled(state.commands.canRedo);
     this.playButton.setText(editing ? '▶  Play' : '■  Stop');
     this.playButton.variant = editing ? 'primary' : 'danger';
-    this.playButton.setEnabled(scene !== undefined);
+    this.playButton.setEnabled(from !== undefined);
     this.playButton.invalidate();
     this.pauseButton.setText(state.paused ? '▶  Resume' : '❚❚  Pause');
     this.pauseButton.setEnabled(!editing);
     this.pauseButton.invalidate();
     this.projectName.setEnabled(editing);
-    this.sceneButton.setEnabled(editing);
+    // Stays usable in play mode: it only says which scene the editor looks at, which is the same
+    // thing a double-click in the Assets panel does – and that one was never blocked. The run is
+    // not touched by it; a running scene is swapped by `Load scene`, not from here.
+    this.sceneSelect.setEnabled(true);
     this.status.setText(SAVE_LABELS[state.saveStatus]);
     this.status.setStyle(state.saveStatus === 'error' || state.saveStatus === 'conflict' ? 'body' : 'small');
   }

src/editor/settings.ts

@@ -0,0 +1,47 @@
+/**
+ * The handful of things the editor remembers about *how you work*, as opposed to what you built.
+ * They belong to this browser, not to the project: a second person opening the same project has
+ * their own answer, and none of this may end up in a diff of the project file.
+ *
+ * Storage is best effort. A browser that refuses `localStorage` (private mode, a blocked origin)
+ * gets the defaults and keeps working – a setting is not worth an error.
+ */
+
+const KEY = 'game-maker.editor';
+
+/** Which scene Play starts: the one named as the start scene, or whatever is open right now. */
+export type PlayFrom = 'start' | 'active';
+
+export interface EditorSettings {
+  playFrom: PlayFrom;
+}
+
+const DEFAULTS: EditorSettings = { playFrom: 'start' };
+
+let current: EditorSettings | null = null;
+
+export function settings(): EditorSettings {
+  if (current) return current;
+  current = { ...DEFAULTS };
+  try {
+    const raw = window.localStorage.getItem(KEY);
+    const read: unknown = raw ? JSON.parse(raw) : null;
+    if (read && typeof read === 'object') {
+      const { playFrom } = read as Partial<EditorSettings>;
+      if (playFrom === 'start' || playFrom === 'active') current.playFrom = playFrom;
+    }
+  } catch {
+    // Unreadable or refused: the defaults stand.
+  }
+  return current;
+}
+
+export function updateSettings(change: Partial<EditorSettings>): void {
+  const next = { ...settings(), ...change };
+  current = next;
+  try {
+    window.localStorage.setItem(KEY, JSON.stringify(next));
+  } catch {
+    // Not stored is not a failure worth reporting; the session keeps the value either way.
+  }
+}

src/editor/state.ts

@@ -11,10 +11,12 @@ import {
   findObjectHolder,
   findScene,
   type Project,
+  startScene,
 } from '../core/model/Project';
 import { deserializeProject, serializeProject } from '../core/store/serialize';
 import { requestRender } from '../ui';
 import { consoleLog } from './console/log';
+import { type PlayFrom, settings, updateSettings } from './settings';
 
 export type Selection =
   | { kind: 'asset'; assetId: string }
@@ -193,6 +195,30 @@ export class EditorState {
     return findScene(this.project, this.project.activeSceneId);
   }
 
+  /** The play-mode copy while a run is on, else null – what a scene swap rebuilds its world from. */
+  get runtimeProject(): Project | null {
+    return this.mode === 'play' ? this.project : null;
+  }
+
+  /**
+   * Which scene Play builds its world from. Two answers rather than one, because the project has
+   * two: where the game begins, and what you are working on this minute. The switch says which
+   * question Play is asking; without a start scene the two are the same thing anyway.
+   */
+  sceneToPlay(): SceneAsset | undefined {
+    return this.playFrom === 'active' ? this.activeScene : startScene(this.project);
+  }
+
+  get playFrom(): PlayFrom {
+    return settings().playFrom;
+  }
+
+  setPlayFrom(playFrom: PlayFrom): void {
+    if (playFrom === this.playFrom) return;
+    updateSettings({ playFrom });
+    this.emit('mode');
+  }
+
   /**
    * Runs a command through the undo stack. Returns false (and sets a notice) when invalid.
    * In play mode the command changes the runtime copy only (see `enterPlay`).

src/runtime/World.ts

@@ -39,6 +39,12 @@ export class World {
   stopRequested = false;
   /** Set by a behavior (Restart game node); the editor restarts the scene on the next tick. */
   restartRequested = false;
+  /**
+   * Set by a behavior (Load scene node): the scene to swap to once this tick is over, `''` for
+   * none asked. Asked twice in a tick, the last one wins – a flag, not a queue, because loading
+   * two scenes in a row is not a thing anyone means.
+   */
+  loadSceneRequested = '';
   /**
    * Mouse state in world units, fed by the game view (the only place with the camera
    * transform). `pressed`/`released` last for one tick, like the keyboard edges.

src/runtime/systems/Audio.ts

@@ -7,13 +7,21 @@
  * holding the project. Everything below this line only knows bytes and voices.
  */
 export interface AudioSink {
-  /** Starts one voice. `volume` is final (0–1) and every call overlaps whatever is already playing. */
-  play(file: string, volume: number, loop: boolean): void;
+  /**
+   * Starts one voice. `volume` is final (0–1) and every call overlaps whatever is already playing.
+   * `owner` is the behavior the sound belongs to: when that behavior is destroyed the voice goes
+   * with it, because a sound is something a thing in the world is doing, not a thing of its own.
+   * `''` means the voice has no owner and outlives whatever started it.
+   */
+  play(file: string, volume: number, loop: boolean, owner: string): void;
   /** Silences every voice of that file; `''` silences all of them. */
   stop(file: string): void;
+  /** Silences every voice started by that owner – what destroying a behavior does. */
+  stopOwner(owner: string): void;
 }
 
 export const SILENT_AUDIO: AudioSink = {
   play: () => {},
   stop: () => {},
+  stopOwner: () => {},
 };

src/runtime/systems/BehaviorSystem.ts

@@ -18,7 +18,7 @@ import {
 import { stepKeyRepeat } from '../../core/graph/keyRepeat';
 import { graphLookup } from '../../core/model/graphLookup';
 import { type PropertyBag, textOf } from '../../core/model/Properties';
-import { findAssetOf, type Project } from '../../core/model/Project';
+import { findAssetOf, findScene, type Project } from '../../core/model/Project';
 import { soundFileOf } from '../../core/model/sound';
 import { parentChain } from '../../core/model/walk';
 import { resolveAssetProps } from '../../core/properties/resolve';
@@ -85,10 +85,13 @@ export function addGameSystems(
   input: Input,
   shouldRun?: (entity: Entity) => boolean,
   audio: AudioSink = SILENT_AUDIO,
+  /** What a scene swap hands over from the world before it (see `BehaviorSystem.carryOver`). */
+  carried: Readonly<SharedState> | null = null,
 ): BehaviorSystem {
   const collisions = new CollisionSystem();
   const animations = new AnimationSystem();
   const behaviors = new BehaviorSystem(project, input, { collisions, animations, audio }, shouldRun);
+  if (carried) behaviors.carryOver(carried);
   world.addSystem(collisions);
   world.addSystem(animations);
   world.addSystem(behaviors);
@@ -112,6 +115,8 @@ export class BehaviorSystem implements System {
   private instances: Instance[] = [];
   private world: World | null = null;
   private sharedState: SharedState = emptySharedState();
+  /** Handed over by the previous world of the same run; consumed by the next `start`. */
+  private carried: SharedState | null = null;
   /** Moves whose events are still owed; drained by `flushStateChanges` between exec flows. */
   private pending: StateChange[] = [];
   /** The declarations as last read; an edit is what differs from these (see `syncDeclarations`). */
@@ -152,23 +157,43 @@ export class BehaviorSystem implements System {
     this.declared = now;
   }
 
+  /**
+   * Takes over the shared state of the world before it – the values of the Variables assets and
+   * where the global machines stand – instead of building a fresh one. That is what belongs to the
+   * run rather than to the scene, and it is what makes a score survive the next level. Call before
+   * the world is started.
+   */
+  carryOver(state: Readonly<SharedState>): void {
+    // The bags themselves travel as they are – a running value is the point – while the map around
+    // them is copied, so the old system cannot still be writing into what the new one reads.
+    this.carried = { variables: { ...state.variables }, states: { ...state.states } };
+  }
+
   start(world: World): void {
     this.world = world;
     this.instances = [];
     this.pending = [];
+    const carried = this.carried;
+    this.carried = null;
     // Built fresh for every run: a shared variable starts at its declared value each time Play does.
-    this.sharedState = buildSharedState(this.project);
+    this.sharedState = carried ?? buildSharedState(this.project);
     this.declared = declaredVariables(this.project);
     // A `global` machine is entered once for the whole run, before the first instance exists – so
     // the move is queued here and every instance the first sync creates hears that one entry.
-    for (const [machineId, start] of Object.entries(this.sharedState.states))
-      if (start) this.pending.push({ machineId, from: '', to: start, source: null });
+    // Not after a scene swap: the machine is already standing where it stands, and announcing the
+    // entry again would be a move that never happened.
+    if (!carried)
+      for (const [machineId, start] of Object.entries(this.sharedState.states))
+        if (start) this.pending.push({ machineId, from: '', to: start, source: null });
     this.syncInstances(world);
   }
 
   /** Tearing the world down (stop, restart) destroys every object, so each instance hears it. */
   stop(): void {
-    for (const instance of this.instances) this.finish(instance);
+    for (const instance of this.instances) {
+      this.finish(instance);
+      this.audio.stopOwner(instance.entity.id);
+    }
     this.instances = [];
     this.pending = [];
     this.world = null;
@@ -291,7 +316,12 @@ export class BehaviorSystem implements System {
     const gone = this.instances.filter((i) => !world.entities.includes(i.entity));
     if (gone.length === 0) return;
     this.instances = this.instances.filter((i) => !gone.includes(i));
-    for (const instance of gone) this.finish(instance);
+    for (const instance of gone) {
+      this.finish(instance);
+      // After `On destroy`, not before: that graph may still say something, and a sound it wants
+      // to outlive the object says so itself by being detached.
+      this.audio.stopOwner(instance.entity.id);
+    }
   }
 
   private finish(instance: Instance): void {
@@ -454,12 +484,12 @@ export class BehaviorSystem implements System {
    * node's, and its loop flag unless the node overrules it either way. The asset is what a sound
    * *is* – a node that says nothing therefore plays it as the project describes it.
    */
-  private playSound(assetId: string, volume: number, loop: boolean | undefined): void {
+  private playSound(assetId: string, volume: number, loop: boolean | undefined, owner: string): void {
     const asset = findAssetOf(this.project, assetId, 'sound');
     if (!asset?.file) return;
     const props = resolveAssetProps(this.project, asset);
     const level = Math.max(0, Math.min(1, volumeOf(props) * volume));
-    this.audio.play(asset.file, level, loop ?? loopsOf(props));
+    this.audio.play(asset.file, level, loop ?? loopsOf(props), owner);
   }
 
   /** Runs graph code for one instance; an exception stops play and is logged, never thrown. */
@@ -521,7 +551,8 @@ export class BehaviorSystem implements System {
         // eslint-disable-next-line no-console
         console.log(`[behavior ${entity.name}]`, message);
       },
-      playSound: (assetId, volume, loop) => this.playSound(assetId, volume, loop),
+      playSound: (assetId, volume, loop, detached) =>
+        this.playSound(assetId, volume, loop, detached ? '' : entity.id),
       // An id nothing resolves stops nothing, rather than silencing the whole run by accident;
       // only the empty id means "everything", which is what the node promises.
       stopSound: (assetId) => {
@@ -533,6 +564,12 @@ export class BehaviorSystem implements System {
         const found = resolve(target);
         if (found) world.remove(found);
       },
+      loadScene: (sceneId) => {
+        // An id naming no scene is dropped here rather than half way through the swap, where the
+        // run would already have torn down the world it is still standing in.
+        if (findScene(this.project, sceneId)) world.loadSceneRequested = sceneId;
+        else world.log.push(`Load scene: no scene ${sceneId}`);
+      },
       stop: () => {
         world.stopRequested = true;
       },

src/viewport/SoundLibrary.ts

@@ -2,9 +2,11 @@ import type { AudioSink } from '../runtime/systems/Audio';
 
 const DEFAULT_BASE = '/game-maker/data/sounds';
 
-/** One sound in flight: its source, its own gain, and the file it came from. */
+/** One sound in flight: its source, its own gain, the file it came from and who is making it. */
 interface Voice {
   file: string;
+  /** The behavior this voice belongs to; `''` for one nothing owns (see `AudioSink.play`). */
+  owner: string;
   source: AudioBufferSourceNode;
   gain: GainNode;
 }
@@ -42,37 +44,46 @@ export class SoundLibrary implements AudioSink {
    */
   private readonly stops = new Map<string, number>();
   private allStops = 0;
+  /** The same counter per owner, for the same reason – a behavior destroyed while bytes were in
+   * flight must not be given its voice afterwards. */
+  private readonly ownerStops = new Map<string, number>();
 
   /** Where the files are served from. Set once at start-up, before anything plays. */
   configure(base: string): void {
     this.base = base.replace(/\/+$/, '');
   }
 
-  play(file: string, volume: number, loop: boolean): void {
+  play(file: string, volume: number, loop: boolean, owner = ''): void {
     if (file === '' || this.failed.has(file)) return;
     const buffer = this.buffers.get(file);
     if (buffer) {
-      this.start(file, buffer, volume, loop);
+      this.start(file, buffer, volume, loop, owner);
       return;
     }
-    const askedAt = this.stopsFor(file);
+    const askedAt = this.cancelsFor(file, owner);
     void this.load(file).then((loaded) => {
-      // A `Stop sound` that happened while the bytes were in flight has to win, or a sound the
-      // graph already silenced would start afterwards and play on its own.
-      if (loaded && this.stopsFor(file) === askedAt) this.start(file, loaded, volume, loop);
+      // A `Stop sound` – or the destruction of the behavior that asked – that happened while the
+      // bytes were in flight has to win, or a sound the run already ended would start afterwards.
+      if (loaded && this.cancelsFor(file, owner) === askedAt) this.start(file, loaded, volume, loop, owner);
     });
   }
 
   stop(file: string): void {
-    const going = file === '' ? this.voices : this.voices.filter((voice) => voice.file === file);
-    for (const voice of going) voice.source.stop();
-    // `onended` removes each one, but not before the next frame – the count has to be right now.
-    this.voices = this.voices.filter((voice) => !going.includes(voice));
-    this.stopped += going.length;
+    this.silence(file === '' ? this.voices : this.voices.filter((voice) => voice.file === file));
     if (file === '') this.allStops += 1;
     else this.stops.set(file, (this.stops.get(file) ?? 0) + 1);
   }
 
+  /**
+   * Every voice of one behavior. An empty owner is deliberately no one: those are the voices that
+   * were started to outlive their behavior, so "stop the owner" must not reach them.
+   */
+  stopOwner(owner: string): void {
+    if (owner === '') return;
+    this.silence(this.voices.filter((voice) => voice.owner === owner));
+    this.ownerStops.set(owner, (this.ownerStops.get(owner) ?? 0) + 1);
+  }
+
   /** Every voice at once: what ending a run does, so nothing outlives the world that started it. */
   stopAll(): void {
     this.stop('');
@@ -137,7 +148,14 @@ export class SoundLibrary implements AudioSink {
     return work;
   }
 
-  private start(file: string, buffer: AudioBuffer, volume: number, loop: boolean): void {
+  /** Ends these voices now: `onended` only fires next frame and the counts have to be right here. */
+  private silence(going: Voice[]): void {
+    for (const voice of going) voice.source.stop();
+    this.voices = this.voices.filter((voice) => !going.includes(voice));
+    this.stopped += going.length;
+  }
+
+  private start(file: string, buffer: AudioBuffer, volume: number, loop: boolean, owner: string): void {
     const context = this.audio();
     // A context starts suspended where the page has not been interacted with; without this the
     // voice would run silently and end as if it had played.
@@ -149,7 +167,7 @@ export class SoundLibrary implements AudioSink {
     gain.gain.value = Math.max(0, Math.min(1, volume));
     this.last = { file, volume: gain.gain.value, loop };
     source.connect(gain).connect(context.destination);
-    const voice: Voice = { file, source, gain };
+    const voice: Voice = { file, owner, source, gain };
     source.onended = () => {
       this.voices = this.voices.filter((other) => other !== voice);
     };
@@ -162,9 +180,9 @@ export class SoundLibrary implements AudioSink {
     return (this.context ??= new AudioContext());
   }
 
-  /** Stops this file has seen, its share of the "everything" ones included. */
-  private stopsFor(file: string): number {
-    return (this.stops.get(file) ?? 0) + this.allStops;
+  /** Everything that would have silenced this file for this owner, counted rather than flagged. */
+  private cancelsFor(file: string, owner: string): number {
+    return (this.stops.get(file) ?? 0) + this.allStops + (this.ownerStops.get(owner) ?? 0);
   }
 }