Clear the pen layer when runtime dispose happens.

src/engine/runtime.js

@@ -626,6 +626,14 @@ class Runtime extends EventEmitter {
         return 'RUNTIME_STARTED';
     }
 
+    /**
+     * Event name when the runtime dispose has been called.
+     * @const {string}
+     */
+    static get RUNTIME_DISPOSED () {
+        return 'RUNTIME_DISPOSED';
+    }
+
     /**
      * Event name for reporting that a block was updated and needs to be rerendered.
      * @const {string}
@@ -1547,6 +1555,7 @@ class Runtime extends EventEmitter {
         this.stopAll();
         this.targets.map(this.disposeTarget, this);
         this._monitorState = OrderedMap({});
+        this.emit(Runtime.RUNTIME_DISPOSED);
         // @todo clear out extensions? turboMode? etc.
 
         // *********** Cloud *******************

src/extensions/scratch3_pen/index.js

@@ -67,6 +67,7 @@ class Scratch3PenBlocks {
         this._onTargetMoved = this._onTargetMoved.bind(this);
 
         runtime.on('targetWasCreated', this._onTargetCreated);
+        runtime.on('RUNTIME_DISPOSED', this.clear.bind(this));
     }
 
     /**

test/unit/engine_runtime.js

@@ -239,3 +239,14 @@ test('setCompatibilityMode does not restart if it was not running', t => {
     t.equal(started, false);
     t.end();
 });
+
+test('Disposing the runtime emits an event', t => {
+    let disposed = false;
+    const rt = new Runtime();
+    rt.addListener('RUNTIME_DISPOSED', () => {
+        disposed = true;
+    });
+    rt.start();
+    t.equal(disposed, true);
+    t.end();
+});