I have my menu set to auto-hide and when I right-click on a menu for something therein, it often picks up artifacts from the menu until I show/hide the panel again. I don't imagine this will make much sense, so I have a screenshot if I could put it somewhere....
You can create a new attachment to this bug for the screenshot (http://bugzilla.xfce.org/attachment.cgi?bugid=348&action=enter). It might indeed be helpful ;) What gtk and XOrg versions are you using?
Created attachment 67 Panel screenshot showing a menu from a systemtray icon I couldn't quite get the import programme to go all the way to the bottom of the screen, so this is cut off just a bit, but you can still see what I mean.
This sounds to me like an XOrg (+ composite) bug. Don't you think so?
GTK 2.4.9 and X.Org 6.8.0 Composite problem? Huh. I have no idea how the panel redraws itself when hiding etc. so I really don't know where this problem lies.
Sorry, I should have explained. Xfce doesn't do the redrawing, that's gtk / X. There have been several reports about refresh/redraw problems with the new XOrg (6.8), either with or without the composite extension. I think this is related.
Definitely an XOrg/Composite issue. Hopefully they'll fix these glitches soon. Other than that composite rocks ;)