window-list: Stop monitoring drag operation if window list is destroyed

If a user is in the middle of a drag in the window list and the
window list associated with the drag gets destroyed, the drag
monitor gets leaked.

Later when the drag motion is processed, spew goes to the log:

clutter_actor_contains: assertion 'CLUTTER_IS_ACTOR (self)' failed

Examples of triggers for this bug:

- The monitor topology changes
- The screen gets locked during the drag

This commit fixes the spew and the leak by ensuring any pending
drag monitoring is disabled when the window lists are destroyed.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/145>

extensions/window-list/extension.js

@@ -786,9 +786,9 @@ class WindowList extends St.Widget {
             });
 
         this._dragBeginId = Main.xdndHandler.connect('drag-begin',
-            this._onDragBegin.bind(this));
+            this._monitorDrag.bind(this));
         this._dragEndId = Main.xdndHandler.connect('drag-end',
-            this._onDragEnd.bind(this));
+            this._stopMonitoringDrag.bind(this));
         this._dragMonitor = {
             dragMotion: this._onDragMotion.bind(this),
         };
@@ -1016,11 +1016,11 @@ class WindowList extends St.Widget {
         }
     }
 
-    _onDragBegin() {
+    _monitorDrag() {
         DND.addDragMonitor(this._dragMonitor);
     }
 
-    _onDragEnd() {
+    _stopMonitoringDrag() {
         DND.removeDragMonitor(this._dragMonitor);
         this._removeActivateTimeout();
     }
@@ -1094,6 +1094,7 @@ class WindowList extends St.Widget {
 
         global.display.disconnect(this._fullscreenChangedId);
 
+        this._stopMonitoringDrag();
         Main.xdndHandler.disconnect(this._dragBeginId);
         Main.xdndHandler.disconnect(this._dragEndId);