I don't really know how to describe this problem that's why i made a video Reproducible: Always
Check if turning off instant preview under view->instant preview helps?
(In reply to wolthera from comment #1) > Check if turning off instant preview under view->instant preview helps? There's no "instant preview" there
Then please update to Krita 3.0, there's a ton of bugfixes between 3.0 and 2.9. We will not fix bugs in 2.9 anymore.