overview.js: Don't hide the global.window_group while animating

Hiding it also hides our background actor. During animation the temporary
background we create to show in the overview fades out leaving us with only
the black stage visible. This leads to the appearance of flickering.

Author: JosephMcc

js/ui/overview.js

@@ -275,17 +275,8 @@ Overview.prototype = {
         this._coverPane.connect('event', () => true);
         this._coverPane.hide();
 
-        // All the the actors in the window group are completely obscured,
-        // hiding the group holding them while the Overview is displayed greatly
-        // increases performance of the Overview especially when there are many
-        // windows visible.
-        //
-        // If we switched to displaying the actors in the Overview rather than
-        // clones of them, this would obviously no longer be necessary.
-        //
         // Disable unredirection while in the overview
         Meta.disable_unredirect_for_screen(global.screen);
-        global.window_group.hide();
         this._group.show();
 
         this.workspacesView = new WorkspacesView.WorkspacesView();
@@ -456,8 +447,6 @@ Overview.prototype = {
         // Re-enable unredirection
         Meta.enable_unredirect_for_display(global.display);
 
-        global.window_group.show();
-
         this.workspacesView.destroy();
         this.workspacesView = null;