I was using one of the default colour smudge brushes (mixover_*) and when painting it suddenly created a square transparent area into the drawing (spot where I was painting on). I was able to remove it by a few undos. Reproducible: Couldn't Reproduce
Hi Mira-maia, Thanks for your report. It must have been either a temporary update glitch in the canvas, or a glitch in the image composition. Are you using the opengl option?
Heya, sorry my reply was this late. I've noticed this same bug happening on my linux version of krita as well (it's 2.8 tho). I'm not using opengl on that version atleast. On windows with 2.9 I do have opengl on (atleast for now), but I only use it for testing so I've not managed to reproduce the bug over there. It seems to happen with several different brushes too, although rather rarely, around once every three drawings I work on and especially when I work really fast. As far as I can tell it seems to be that the area just isn't displayed on the layer, but it's still actually there. When I paint over the empty spot the 'erased block' becomes visible again.
I think we finally found the conditions where this can happen. Check bug 359760 -- and we're working on a solution. *** This bug has been marked as a duplicate of bug 359760 ***