Expose is too late, everything breaks in KWin, see [1]. With show event
we get the integration working correctly. No idea what the comment was
about, but in my testing on Qt 5.9 this works just fine.

[1] https://mail.kde.org/pipermail/plasma-devel/2017-July/072139.html

Run KRunner on Wayland with maximized windows. Properly positioned,
mentioned issues gone.