Wrong black lines in 4:3 aspect ratio

[marisathewitch]

On 1280*1024 resolution we can found this

[atomheartother]

I first dismissed this issue as a browser-specific quirk I'd have to figure out later but I reproduced it! I'll try and fix it, thanks for your bug report!

[clockfort]

incorrect black bars at 3840x2160 as well (see bottom of image); though, wrong in a different way than the original issue poster.
The same issue seems to happen to me regardless of resolution or aspect ratio, I tested it in a few different window sizes.

Chrome Version 84.0.4147.68 (Official Build) beta (64-bit)