Reproduction:
1) Open application style -> window decorations -> buttons
2) Change size few times
3) Maximize/ unmaximize few times by double-click on header 
4) Change size to minimum possible

Result:
1) Size change takes ~1 second, part of the window filled with black color until its done, than freezes completely.
2) When window size is minimal, content can be bigger than window. Systemsettings starts flickering, even when it is below another window (firefox as example)

supportInformation:
https://paste.kde.org/pem1pnp3h