workspacesView: Work around spurious allocation changes

For some reason, people are still seeing those after commit d5ebd8c8.
While this is something we really should figure out, we can work around
the issue by keeping the view actors hidden until the update is complete.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1065

js/ui/workspacesView.js

@@ -638,10 +638,15 @@ var WorkspacesDisplay = class {
                                                this._scrollValueChanged.bind(this));
             }
 
+            // HACK: Avoid spurious allocation changes while updating views
+            view.actor.hide();
+
             this._workspacesViews.push(view);
             Main.layoutManager.overviewGroup.add_actor(view.actor);
         }
 
+        this._workspacesViews.forEach(v => v.actor.show());
+
         this._updateWorkspacesFullGeometry();
         this._updateWorkspacesActualGeometry();
     }