when dragging one of the external plugins (mixer, menu, mailwatch) to another
panel it only shows up after the panel is restarted.It occupies space on the new
panel but it's not there.

This is printed on the console on such a move

(xfce4-panel:28024): Gdk-WARNING **: gdkdrawable-x11.c:904 drawable is not a
pixmap or window

(xfce4-panel:28024): Gdk-WARNING **: gdkdrawable-x11.c:904 drawable is not a
pixmap or window



Reproducible: Always
Steps to Reproduce:
1. 
2. 
3.