Submenus positioned relative to main window instead of parent menu

The QMenu and all it's child menus are positioned relative to the main window. If the compositor does not position the menu exactly as Qt expects the sub-menus are off. See attached screenshot for a case in KWin.

A compositor has a valid interest in positioning a transient surface different than the client indicated through the offset. The client does not know it's own relative position on the output. Thus a relative positioned transient might end up in non-visible area. The compositor applies sanity checks to ensure the content stays visible.

The sub-menus should be a transient surface for the parent menu. That way the offset could be useful for the Wayland compositor again.