QMenu misplaced on Wayland in some multihead arrangements

Menus of Qt applications are misplaced in multi-monitor setups when the monitors are arranged at different heights, and the left-hand monitor's top is lower than the right-hand monitor's.

(See the screenshots. In practice I scale the laptop screen so that the bottom of the screens align, but the issue is not affected by the scaling.)

I can't tell if the bug is in Qt or the kwin compositor, however some of Qt's own compositor examples are also affected (minimal-cpp and multi-screen).