It looks like screenshot tools encode some information about DPI into the PNG files. e.g. if I upload a screenshot from my high-dpi macOS environment to https://www.metadata2go.com/, I see
If I turn off HiDpi, then those metadata fields seem to disappear from the file.
Do you see something similar on Windows @gerhard? If so, maybe there’s a way for us to read it in JS and make a guess about the pixel density.
I also came across this gist which might be useful.