This is a follow up from bug 130841, as I couldn't find a solution for this problem, originally the 2nd bug found and described in it. The bug is easily reproducible using the bug document from that bug. This way the other, kf5 specific bug can be closed, as it's fixed by the pushed patches.
Every time you do the non-resize, the window shrinks by the height of the native menu bar, because of the frame geometry <> client size difference.
Confirmed already in bug 130841.