window-list: Don't unnecessarily add/remove window buttons

Since commit 191c7ccc24, we check whether we already have a window
in the list before re-adding it on MetaWorkspace::window-added.
We can do something similar on MetaWorkspace::window-removed to
avoid some extra work when a window is moved between workspaces
rather than destroyed.

extensions/window-list/extension.js

@@ -217,6 +217,9 @@ const WindowButton = new Lang.Class({
         this._switchWorkspaceId =
             global.window_manager.connect('switch-workspace',
                                           Lang.bind(this, this._updateVisibility));
+        this._workspaceChangedId =
+            this.metaWindow.connect('workspace-changed',
+                                    Lang.bind(this, this._updateVisibility));
         this._updateVisibility();
 
         this._notifyFocusId =
@@ -281,6 +284,7 @@ const WindowButton = new Lang.Class({
     },
 
     _onDestroy: function() {
+        this.metaWindow.disconnect(this._workspaceChangedId);
         global.window_manager.disconnect(this._switchWorkspaceId);
         global.display.disconnect(this._notifyFocusId);
         this._contextMenu.destroy();
@@ -1006,6 +1010,9 @@ const WindowList = new Lang.Class({
             return;
         }
 
+        if (win.get_compositor_private())
+            return; // not actually removed, just moved to another workspace
+
         let children = this._windowList.get_children();
         for (let i = 0; i < children.length; i++) {
             if (children[i]._delegate.metaWindow == win) {