On upgrading to Firefox-91, the background of a right-side scroll bar is no longer properly displayed. The slider is OK, but the background is garbage.

Prior to the -91 upgrade, there was a problem with the background of a large newly-opened blank window being garbage, but it was a one-time thing that resolved as soon as a Firefox window refresh occurred. Only a "default" (old laptop size) portion of the window was accurately displayed. My actual screen is 1920x1080, but I open Firefox in less than full screen. I ignored this earlier problem because it was transient. The -91 scroll bar display issue is permanent. I reverted to a Slackpkg for firefox-87.0 from March 2021 and both problems went away. No other programs with scroll bars and sliders exhibit this problem AFIK. And no other program that opened a large but less than full-screen window (Thunar, LibreOffice, etc) exhibited the earlier problem.

Odd. I'm at 256x1440 but regardless of Firefox 91.0 size, or any other app so far, the scroll bar and it's background are unchanged. I wonder if this issue is Theme based? I wish I could recall the exact instance but I vaguely remember an Alien Bob documentation that a few years ago noted that (I think it was) Oxygen conflicted with some app or other effect. Perhaps he can enlighten us on this. Display is rather complex, especially under Xorg, and especially with apps that have their own backgrounds and themes in addition to and on top of Desktop's own.

After reverting as in my earlier post, I discovered that the same issue occurs on the Firefox-87 I reverted to, but only when using the Ctrl-F (find) option with "Highlight All" selected. I don't usually use "find" in Firefox, but I was chasing the Thunderbird issue I also posted about and was looking for older versions in the Slackware Changelog. When I turned off "Highlight All" the problem disappeared! So this is cosmetic, not important. I'll stick with the reversion, since the partial screen update that appeared somewhere between last March and now -still cosmetic, but more annoying- seems to have gone away with the reversion.