dnd: Cancel the animation on drag actor destruction

If the drag actor is destroyed before the animation
callback is called, the callback is never called and
we're sticked with dnd grabing the events after we
dropped the target.

https://bugzilla.gnome.org/show_bug.cgi?id=757676

js/ui/dnd.js

@@ -571,19 +571,12 @@ const _Draggable = new Lang.Class({
             return;
         }
 
-        this._animationInProgress = true;
+        this._animateDragEnd(eventTime,
-        // No target, so snap back
-        Tweener.addTween(this._dragActor,
                              { x: snapBackX,
                                y: snapBackY,
                                scale_x: snapBackScale,
                                scale_y: snapBackScale,
-                           opacity: this._dragOrigOpacity,
                                time: SNAP_BACK_ANIMATION_TIME,
-                           transition: 'easeOutQuad',
-                           onComplete: this._onAnimationComplete,
-                           onCompleteScope: this,
-                           onCompleteParams: [this._dragActor, eventTime]
                              });
     },
 
@@ -596,18 +589,44 @@ const _Draggable = new Lang.Class({
         this._dragActor.set_scale(restoreScale, restoreScale);
         this._dragActor.opacity = 0;
 
+        this._animateDragEnd(eventTime,
+                             { time: REVERT_ANIMATION_TIME });
+    },
+
+    _animateDragEnd: function (eventTime, params) {
         this._animationInProgress = true;
-        Tweener.addTween(this._dragActor,
+
-                         { opacity: this._dragOrigOpacity,
+        // finish animation if the actor gets destroyed
-                           time: REVERT_ANIMATION_TIME,
+        // during it
-                           transition: 'easeOutQuad',
+        this._dragActorDestroyId =
-                           onComplete: this._onAnimationComplete,
+            this._dragActor.connect('destroy',
-                           onCompleteScope: this,
+                                    Lang.bind(this, this._finishAnimation));
-                           onCompleteParams: [this._dragActor, eventTime]
+
-                         });
+        params['opacity']          = this._dragOrigOpacity;
+        params['transition']       = 'easeOutQuad';
+        params['onComplete']       = this._onAnimationComplete;
+        params['onCompleteScope']  = this;
+        params['onCompleteParams'] = [this._dragActor, eventTime];
+
+        // start the animation
+        Tweener.addTween(this._dragActor, params)
+    },
+
+    _finishAnimation : function () {
+        if (!this._animationInProgress)
+            return
+
+        this._animationInProgress = false;
+        if (!this._buttonDown)
+            this._dragComplete();
+
+        global.screen.set_cursor(Meta.Cursor.DEFAULT);
     },
 
     _onAnimationComplete : function (dragActor, eventTime) {
+        dragActor.disconnect(this._dragActorDestroyId);
+        this._dragActorDestroyId = 0;
+
         if (this._dragOrigParent) {
             Main.uiGroup.remove_child(this._dragActor);
             this._dragOrigParent.add_actor(this._dragActor);
@@ -616,12 +635,9 @@ const _Draggable = new Lang.Class({
         } else {
             dragActor.destroy();
         }
-        global.screen.set_cursor(Meta.Cursor.DEFAULT);
-        this.emit('drag-end', eventTime, false);
 
-        this._animationInProgress = false;
+        this.emit('drag-end', eventTime, false);
-        if (!this._buttonDown)
+        this._finishAnimation();
-            this._dragComplete();
     },
 
     _dragComplete: function() {