workspace: Ignore new windows that appear while the overview is closing

This can easily happen if an app launches and maps a window faster than
the overview closes. We don't want the window clone appearing and then
suddenly disappearing in that case.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1440>

js/ui/workspace.js

@@ -1280,6 +1280,7 @@ class Workspace extends St.Widget {
     }
 
     _windowAdded(metaWorkspace, metaWin) {
+        if (!Main.overview.closing)
             this._doAddWindow(metaWin);
     }
 
@@ -1288,7 +1289,7 @@ class Workspace extends St.Widget {
     }
 
     _windowEnteredMonitor(metaDisplay, monitorIndex, metaWin) {
-        if (monitorIndex == this.monitorIndex)
+        if (monitorIndex === this.monitorIndex && !Main.overview.closing)
             this._doAddWindow(metaWin);
     }