Sadly, to reproduce this bug, you'll need a reddit account, because I have not attempted to create a minimal reproducer for this. Try to post a comment in any subreddit on reddit.com. When editing your comment, select some text. The selected text should be indicated by a blue background, but instead no selection is visible. The text really does get selected, so this is only cosmetic. This worked fine with GTK 3.
I can't reproduce this. Was this only reproducible with threaded rendering?
Nope, it's still 100% reproducible for me.
Then I need more info - what specific page it is given they all have significantly different styles, old vs new reddit etc.
Normal/new reddit. The bug occurs on every page in every subreddit, but only in the comment editor, when posting or editing a comment. Actually, testing it now, I see now that the background color displays properly if I wait for a few seconds after selecting the text. It also displays properly if I press Ctrl+A. So this looks like a performance or rendering issue, similar to the problem with the emoji picker on Element, rather than a theme issue.
Ah, so probably redraw then, not performance. And since I can't reliably reproduce it, it worked here too. Great, great. :(
This was mysteriously fixed sometime in the past year.