This was caused by a rendering progression.
<rdar://65535704>
Created attachment 405407 [details] Patch
Created attachment 405408 [details] Before/after comp (dark mode, window inactive)
Comment on attachment 405407 [details] Patch r=me, great explanation!
Committed r265008: <https://trac.webkit.org/changeset/265008> All reviewed patches have been landed. Closing bug and clearing flags on attachment 405407 [details].