@roger64: If you mean "holes" in the UI, where you basically see bits of the vanilla UI underneath KOReader, then, yeah, that can happen (on recent-ish FWs especially, >= 5.6.x, IIRC).
I tried taking care of the most glaring ones where possible, but most of it depends on timing and on the screen's reactivity at that specific moment in time (which, itself, depends on a looooot of things), and while in some instances we have a bit of leeway in the timings involved, sometimes, we don't

.