Menu position is wrong when working on secondary screen (xcb)

When I run any Qt 5 application on my primary screen the menu that belongs to a menu bar is displayed directly beneath the relevant entry in the menu bar. If I move the application to my secondary screen, then there is a gap of about 6 pixels between the menu popup-menu and the menubar entry.

This happens on linux (xcb backend), running xmonad as a window manager. I tested this with QtCreator as found in the Qt 5 package, but other apps work the same.