Based on this email [1], it'd be nice if the libxfce4panel API documentation [2] for xfce_panel_plugin_register_menu() could be updated. Here's an excerpt:

BEGIN QUOTE

> 2) registering menu with panel
>
> xfdesktop calls xfce_panel_plugin_register_menu() before each time the
> menu is opened. Is it sufficient to call
> xfce_panel_plugin_register_menu() only when the menu is (re-)created?
>
>   
No you have to call it every time the menu is opened. That function 
probably should have been called *_register_open_menu() or something. It 
allows the panel to properly handle focus, transparency, autohide and 
probably some other things when the menu closes."

END QUOTE

Perhaps the existing phrasing is a little confusing, too. You don't want to "register an open menu"; you want to register a menu opening.

[1] http://foo-projects.org/pipermail/xfce4-dev/2007-April/022965.html
[2] http://www.xfce.org/documentation/4.4/api/libxfce4panel/XfcePanelPlugin.html#xfce-panel-plugin-register-menu