Created attachment 1138 [details]
Screenshot zoomed in on the issue
This problem was broken out from bug 7918.
After changes made in bug 7918, the borders of boxes or buttons appear slightly "thicker" a bit above the bottom left and right corners. This seems to be because the bottom part of the border is meant to be darker, and things are not rendered very smoothly on Linux.
This visual artifact does not appear on Windows or macOS.
Created attachment 1139 [details]
Screenshot (not zoomed) showing the issue