Firefox 100+, Horizontal scrollbar not visible with Frozen

I'm actually having the same issue with Ubuntu 20.04 & 22.04, perhaps that's not a platform specific issue.

It is, but in a weird way: https://www.omgubuntu.co.uk/2022/04/mozilla-firefox-99-features-gtk-overlay-scrollbars

Ubuntu also switched to the same "hover and disapear" scrollbars. Firefox is "using the OS's settings" in both cases.

Thanks for reporting the Ubuntu part, I've added the info to the main ticket: https://tracker.zkoss.org/browse/ZK-5159

the workaround attached to the ticket is not platform based. It checks if the browser failed to allocate space to the scrollbar, and add that spacing if needed. So it should work on ubuntu as well. Let me know if you encounter issues with it on ubuntu.