I'm running a dual 4k monitor KDE Plasma 5.8.5 Desktop in openSUSE Tumbleweed. With Screen Scaling set to 2 (i.e. QT_SCALE_FACTOR=2), Qt apps only work as they should when positioned on the primary display. Drop-down menus don't open properly for Qt apps positioned on the secondary display. As soon as I move the Qt App with non-functional menus back to the primary display, everything works as it should again.

If I turn scaling off (QT_SCALE_FACTOR=1) everything works properly on both monitors.

If I have scaling turned off in plasma, but launch an individual Qt app with a QT_SCALE_FACTOR=2 prefix, then that individual app exhibits the same issue of not working properly on the secondary display.

If I switch which monitor is the primary, with scaling on, Qt Apps will switch which monitor they work on--always working only with whichever display is set to primary.