[gnome-shell] workspaceThumbnail: don't queue unnecessary relayouts

[Stefano Facchini <sfacchini src gnome org>]

commit 786beccca5d3f6252a1aa331453dddfd4c2486fb
Author: Stefano Facchini <stefano facchini gmail com>
Date:   Tue Feb 21 20:46:27 2012 +0100

    workspaceThumbnail: don't queue unnecessary relayouts
    
    Relayouts are expensive and can make the UI laggy.

 js/ui/workspaceThumbnail.js |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/js/ui/workspaceThumbnail.js b/js/ui/workspaceThumbnail.js
index 52b2a74..1d18e02 100644
--- a/js/ui/workspaceThumbnail.js
+++ b/js/ui/workspaceThumbnail.js
@@ -598,6 +598,9 @@ const ThumbnailsBox = new Lang.Class({
     },
 
     _clearDragPlaceholder: function() {
+        if (this._dropPlaceholderPos == -1)
+            return;
+
         this._dropPlaceholderPos = -1;
         this.actor.queue_relayout();
     },
@@ -635,8 +638,10 @@ const ThumbnailsBox = new Lang.Class({
             }
         }
 
-        this._dropPlaceholderPos = workspace;
-        this.actor.queue_relayout();
+        if (this._dropPlaceholderPos != workspace) {
+            this._dropPlaceholderPos = workspace;
+            this.actor.queue_relayout();
+        }
 
         if (workspace == -1)
             return DND.DragMotionResult.CONTINUE;