dnd.js: Fix window freeze on rapid taskbar clicks (#13462) (#13465)

Clean up any existing pointer grab before starting a new one in
_onButtonPress. This prevents pointer grabs from accumulating
when users click rapidly on taskbar icons, which could leave
windows in an unresponsive state.

Author: carsteneu

js/ui/dnd.js

@@ -139,6 +139,12 @@ var _Draggable = new Lang.Class({
         if (Tweener.getTweenCount(actor))
             return false;
 
+        // Clean up any existing grab before starting a new one (fixes #13462)
+        // This prevents pointer grabs from accumulating during rapid clicks
+        if (this._onEventId) {
+            this._ungrabActor(event);
+        }
+
         this._buttonDown = true;
         this._grabActor(event);