scaling broken on fractional scaling enabled systems.
I have a system with two 4K displays, one set to 150% and the other to 200% in gnome.
Opening a 4K image on those shows "50%" scaled on the 150% one, and "33%" on the 200% one.
It is a bit hard to tell what's the correct behavior here, but what gwenview seems to do matches reality a bit better and it seems to adjust the "baseline" so 100% means 100% on the 150% display, and 66% on the 200% one. Meaning viewing a 4K image on the 150% display in "fit" mode doesn't blow up the image and overscales it.
It's a bit hard to explain in words, easier to just try it out yourself.