Editing while playing: play runs on a runtime copy, new Game tab (Scene | Game | Graph)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/runtime/systems/BehaviorSystem.ts

@@ -39,19 +39,30 @@ export class BehaviorSystem implements System {
   start(world: World): void {
     this.world = world;
     this.instances = [];
+    this.syncInstances(world);
+  }
+
+  /**
+   * One instance per behavior entity: entities that appeared (placed while playing) get an
+   * interpreter and their `On start`; instances whose entity is gone are dropped.
+   */
+  private syncInstances(world: World): void {
+    this.instances = this.instances.filter((i) => world.entities.includes(i.entity));
     for (const entity of world.entities) {
-      if (entity.type !== 'behavior') continue;
+      if (entity.type !== 'behavior' || this.instances.some((i) => i.entity === entity)) continue;
       const asset = findAsset(this.project, entity.assetId);
       if (!asset || asset.type !== 'behavior') continue;
       const trace: Trace = { edges: new Map(), nodes: new Map() };
       const interpreter = new GraphInterpreter(asset.graph);
       interpreter.trace = this.traceListener(trace, world);
-      this.instances.push({ entity, interpreter, trace });
+      const instance = { entity, interpreter, trace };
+      this.instances.push(instance);
+      this.run(instance, (i, ctx) => i.fire('OnStart', ctx), 0);
     }
-    for (const instance of this.instances) this.run(instance, (i, ctx) => i.fire('OnStart', ctx), 0);
   }
 
   update(world: World, dt: number): void {
+    this.syncInstances(world);
     const keyOf = (node: { params: Record<string, unknown> }) => String(node.params.key ?? '');
     for (const instance of this.instances) {
       if (!world.entities.includes(instance.entity)) continue; // destroyed during play