I'm on Windows 7 64bit, commit 55b069a

Sometimes it happens that if you are fast enough drawing consecutive strokes, so that the next stroke is done before the preview of the current one is finished, you interrupt the preview rendering of that stroke and it disappears until the full/final version is rendered.

I've found a way to somewhat consistently test this:

1. Open an A4 (300ppi) document (or a big enough doc depending on the PC performances)
2. Select possibly big/slow brush (i was using block_mix_tilt but any brush, big enough to the point where the preview mode isn't instant should do)
3. Hit backspace to fill with white
4. Without waiting do two/multiple fast consecutive fast strokes

It should happen that one or two of the first strokes disappear until the full render is ready.