Using Menus under XFCE is inconvenient, the MainMenu is and can be drawn atop XFCE's Top Panel, but all submenus will be drawn below.
Please note that there is also a shadow effect which confirms our menus are "below".
I think the MainMenu is able to float atop because they are both set (checked with xprop) to _NET_WM_WINDOW_TYPE_DOCK which could be argued is correct but inconvenient.
Probably the best solution for XFCE (and similar setups) is for both MainMenu and SubMenus to respect the Top Panel or Bar.
Using Menus under XFCE is inconvenient, the MainMenu is and can be drawn atop XFCE's Top Panel, but all submenus will be drawn below.
Please note that there is also a shadow effect which confirms our menus are "below".
I think the MainMenu is able to float atop because they are both set (checked with xprop) to
_NET_WM_WINDOW_TYPE_DOCKwhich could be argued is correct but inconvenient.Probably the best solution for XFCE (and similar setups) is for both MainMenu and SubMenus to respect the Top Panel or Bar.