Visual artifacts when using sub.TextFile

[Frechdachs]

If I use sub.TextFile to burn in an .ass and seek to a position where no subtitles are shown in the preview and hitting F5 a few times then artifacts will appear. I can't remember it ever happening when first hitting F5 to open the preview after starting vsedit. The artifacts disappear again if I seek to a position where subtitles are shown, after that seeking to a position with no subtitles won't make the artifacts appear again, unless I hit F5 again a few times.

Minimal example script

import vapoursynth as vs
core = vs.get_core()
src = core.ffms2.Source("video.mkv")
out = core.sub.TextFile(src, "subtitles.ass")
out.set_output()

Example Image

https://abload.de/img/fm06nkkx.png

[Aleksey Lyashin]

I confirm this bug, but it is clearly a bug in sub.TextFile(), not the editor.