Added devicePixelRatio to shadow's radius.
Check the shadow of items on the desktop.
Compare scale 100% vs big display scale (e.g. 300%).
Before: Shadow is invisible.
After: Visible shadow at every display scale.
Before vs After
Scale 300% and 100%
Enclosed in Math.round()
Although works fine without it.
Oh gosh, are shadows not scale-aware anywhere? We'd have to replicate this code in a lot of places... might it make more sense to fix this in Qt itself?
Shadows look fine in wayland. Like many other elements, the problem is with X11.