[gnome-shell] workspaceThumbnail: Only create thumbnails once

[Marge Bot <marge-bot src gnome org>]

commit 0e917c3dbf81eecd3d21f9225652eb1ff13e585a
Author: Sebastian Keller <skeller gnome org>
Date:   Thu Apr 29 16:07:21 2021 +0200

    workspaceThumbnail: Only create thumbnails once
    
    `ThumbnailsBox` listens for the `showing` signal from the `Overview` to
    create its thumbnails and destroys them on the `hide` signal. Since the
    `showing` signal can be emitted multiple times when switching between
    the shown and hidden state without ever fully completing the transition,
    this will cause `_createThumbnails` to be called multiple times, each
    time adding another set of workspaces.
    
    Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3819
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1828>

 js/ui/workspaceThumbnail.js | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/js/ui/workspaceThumbnail.js b/js/ui/workspaceThumbnail.js
index 6c5a2bee4a..3a7d916ca2 100644
--- a/js/ui/workspaceThumbnail.js
+++ b/js/ui/workspaceThumbnail.js
@@ -993,6 +993,9 @@ var ThumbnailsBox = GObject.registerClass({
     }
 
     _createThumbnails() {
+        if (this._thumbnails.length > 0)
+            return;
+
         let workspaceManager = global.workspace_manager;
 
         this._nWorkspacesNotifyId =