[gnome-shell/gnome-3-28] workspaceThumbnail: only update _porthole if the overview is visible

[Marco Trevisan <marcotrevi src gnome org>]

commit b476e851b709445f8c0fb0eb7ce34454318cad6d
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Fri Jan 19 16:31:01 2018 +0100

    workspaceThumbnail: only update _porthole if the overview is visible
    
    Otherwise it happens that porthole is computed again after that the
    overlay is hidden (triggered by a layout reallocation) and thus not
    regenerated again afterwards.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=792687
    
    
    (cherry picked from commit 5fcf40b973286fd442148ee3348becd493c8206d)

 js/ui/workspaceThumbnail.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/js/ui/workspaceThumbnail.js b/js/ui/workspaceThumbnail.js
index 78fac80dd..c1b4bddc6 100644
--- a/js/ui/workspaceThumbnail.js
+++ b/js/ui/workspaceThumbnail.js
@@ -1168,7 +1168,7 @@ var ThumbnailsBox = new Lang.Class({
     // The "porthole" is the portion of the screen that we show in the
     // workspaces
     _ensurePorthole() {
-        if (!Main.layoutManager.primaryMonitor)
+        if (!Main.layoutManager.primaryMonitor || !Main.overview.visible)
             return false;
 
         if (!this._porthole)