Bumping here because I never had this issue on desktop, but this week it started out of nowhere.
If my browser window is not a specific width or wider, there is a horizontal scrollbar for just a few pixels. It is present at any smaller width, despite the forum display scaling/shrinking with a smaller window.
Chrome - If the window is not 1964px or wider
Version 94.0.4606.61 (Official Build) (64-bit)
Edge - If the window is not 2160px or wider (the width of my display)
Version 94.0.992.31 (Official build) (64-bit)
Display resolution is 2160x3840 (4k portrait mode)
Again, only in dark mode.