Comment 7 for bug 1397095

This issue is triggered by the following gtk_widget_get_snapshot call:

http://git.xfce.org/xfce/xfce4-panel/tree/plugins/tasklist/tasklist-widget.c#n3032

Requirements for this glitch to occur when dragging a window button:

- panel placed on top
- show button labels
- any external panel plugin placed leftmost within the panel