Apparently this was introduced to prevent the window losing focus if
https://bugs.kde.org/show_bug.cgi?id=310476 is to be believed. However
on X the window does not lose focus without it.
Furthermore it actually improves keyboard navigation on X, which after a
child menu was closed stopped working.
On Wayland this prevents things working at all.
BUG:420980
FIXED-IN:5.23