appDisplay: Consider drag events during page-switching as invalid

While there's an ongoing animation to switch pages, we should ignore all
drag events as those will end up moving the drag item to an unwanted
position.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2348>

js/ui/appDisplay.js

@@ -798,6 +798,7 @@ var BaseAppView = GObject.registerClass({
 
         // Dragging over invalid parts of the grid cancels the timeout
         if (item === source ||
+            this._adjustment.get_transition('value') !== null ||
             page !== this._grid.currentPage ||
             dragLocation === IconGrid.DragLocation.INVALID ||
             dragLocation === IconGrid.DragLocation.ON_ICON) {