I get this too. I did a git bisect, and have found the offending change:
commit 2789519cf18504fd045b3384f011184ac66b8e89 Author: Chong Yidong <address@hidden>
Date: Sat Nov 7 23:49:08 2009 +0000
* xmenu.c (Fx_popup_menu): Extract event timestamp. Pass it to xmenu_show. (xmenu_show): New arg. Pass it to create_and_show_popup_menu. (create_and_show_popup_menu): New arg. Pass it to gtk_menu_popup.
I appreciate that emacs is using bzr, not git, but this is what I have set up right now.
I don't have an immediate fix, but I'm confident that the above change is causing the problem.
I will continue to investigate, but I'm new to emacs hacking so please bear with me...