Pride.codes corners are broken in Internet Explorer

[mashedkeyboard]

Yes, I know. Really, IE?

But yeah, it doesn't work even in IE11. The corner gets displayed over the top of the middle of the scrollbar for some reason, rather than being put in the correct place.

See demo here: https://s.codepen.io/mashedkeyboard/debug/gZLvRg/PBrNWNaZVBnA

Yes, I know. Really, IE?

Completely valid. These sorts of things should work on all platforms.
FYI the codepen expired.