sliding-bookmarks-bar.css doesn't seem to work correctly on its own

[LOuroboros]

Title. This userChrome snippet is not working correctly.

It displays a weird black bar on the place where the bookmarks toolbar is originally located.

SlidingBookmarksBar.mp4

For reference, I'm using Mozilla Firefox v86 (32-bit).

[Timvde]

Same response here, I'm afraid: I didn't write the style, and I'm not actively maintaining any. Hell, I'm not even any good at CSS... I'll gladly accept a fix, but I'm unable to help you, sorry.