I have repaint issues when I use fractional display scale in KDE Plasma, watch my screencast please. It explains better.
Two reports about this issue were created in KDE tracker.
KDE developer Hugo Pereira Da Costa said it's a QT bug:
"This is unfortunately an upstream bug. Breeze itself has no control over clip areas. The same issue appears with other widget styles, and in fact there was a duplicated bug about this some weeks ago (which I cannot find again though ...)
This will have to be reported to QT."