The overflow border of a relatively positioned element inside a region is not painted correctly when using flipped writing modes (horizontal-bt and vertical-rl). See attached test-case, all 4 sides of the green border should be visible.
This problem was fixed for non-flipped writing modes in https://bugs.webkit.org/show_bug.cgi?id=124919
Created attachment 218289 [details]
Test-case highlighting the issue