I still think a bug report could've gone a different way, as other developers could've looked at it (and they would have more stock to persuade the lead). It's part of the PNG specification to be able to set colors on fully transparent pixels, so all of that background should be colored as you want it, period. This…
I suspected that too, but turning instant preview on/off globally or per brush doesn't seem to matter and I get the issue regardless of canvas/brush size, even with small pixel brushes on small canvases I have the issue. Edit: Also I was wrong, I am getting it mid-stroke
Ah, we did actually do work on brush performance: we multi-threaded the brush tip calculation for the pixel brush. I have no idea if it hits the goldilocks zone for you yet. We're hoping to have other brush engines multi-threaded too, but it requires rearranging the existing logic so needs to be done on a case-by-case…