layout: Force size of overviewGroup

The overviewGroup's size is currently determined by the coverPane
actor. That actor is only shown during transitions, so we rely on
ClutterFixedLayout including hidden children in its size request.

That odd behavior is about to change, so we need to make sure the
overview still requests the correct size.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2351>

js/ui/layout.js

@@ -233,6 +233,10 @@ var LayoutManager = GObject.registerClass({
             name: 'overviewGroup',
             visible: false,
             reactive: true,
+            constraints: new Clutter.BindConstraint({
+                source: this.uiGroup,
+                coordinate: Clutter.BindCoordinate.ALL,
+            }),
         });
         this.addChrome(this.overviewGroup);