I noticed in bug 1709037 that with Windows scaling at 125%, the 1px border specified in CSS for this toolbar element was being reported as 0.8px, and this was probably the root cause for the bug - a nested element 14px wide/tall failing to correctly center in an 18px container when it had a 1px border and padding.

I fixed it in this case by centering the element: https://hg.mozilla.org/mozilla-central/rev/f54a65e66247, but I would expect border-width specified this way to be in CSS pixels and not vary with the OS' scaling.

Gijs pointed me at bug 477157 and https://crisal.io/words/2020/06/13/rounding-borders.html which discusses this issue. I'll dupe.