Set gravity survives a re-read of the model during play: World.setSceneProp remembers what the run itself set on the scene and syncFromModel lays it back over the copied scene props, so a project event mid-play no longer resets the gravity to the saved 980; the unit test re-reads the model mid-run and e2e 41c forces a project event right after play starts (#147)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/runtime/World.ts

@@ -37,6 +37,8 @@ export class World {
   readonly entities: Entity[] = [];
   /** The running scene's own properties – read from the model, and written by an animation over the scene. */
   readonly sceneProps: PropertyBag = {};
+  /** What the run itself set on the scene (`setSceneProp`); re-reading the model does not undo it. */
+  private readonly runSceneProps: PropertyBag = {};
   /** Set by a behavior (Stop game node); the editor leaves play mode on the next tick. */
   stopRequested = false;
   /** Set by a behavior (Restart game node); the editor restarts the scene on the next tick. */
@@ -98,7 +100,7 @@ export class World {
     // behavior destroy one of them for good. Play does it (on its throwaway copy); the editor run
     // must not, so there the content stays resolved and `objectRef` writes into the instance edits.
     if (this.bakeContainers && bakeInstances(this.project, scene)) this.structureChanged = true;
-    Object.assign(this.sceneProps, scene.props);
+    Object.assign(this.sceneProps, scene.props, this.runSceneProps);
     const objects = new Map(resolveObjects(this.project, scene).map((o) => [o.id, o]));
     const keep = new Set<Entity>();
     for (const object of objects.values()) {
@@ -115,6 +117,12 @@ export class World {
     }
   }
 
+  /** Sets a scene property for the rest of the run; the scene in the model keeps its own. */
+  setSceneProp(key: string, value: PropertyBag[string]): void {
+    this.sceneProps[key] = value;
+    this.runSceneProps[key] = value;
+  }
+
   /** The entity for a scene object, created when missing; position, name and props re-read. */
   private materialize(object: SceneObject, asset: Asset): Entity {
     let entity = this.findByObjectId(object.id);

src/runtime/physics/PhysicsSystem.ts

@@ -216,8 +216,8 @@ export class PhysicsSystem implements System, CollisionSource {
 
   /** The scene's gravity for the rest of this run, in world units per second squared. */
   setGravity(world: World, gravity: Vec2): void {
-    world.sceneProps.gravityX = gravity.x;
-    world.sceneProps.gravityY = gravity.y;
+    world.setSceneProp('gravityX', gravity.x);
+    world.setSceneProp('gravityY', gravity.y);
     if (this.engine) this.engine.gravity = this.gravity(world);
   }