For anyone else who's been having a headache with the Youtube Full-screen scroll-bar stuff that's been going on for months now, I don't know if its locally on Firefox but I cannot replicate the issue on Chrome or Edge. I am unfamiliar with other browsers to test this on but found that this solution worked.
When the following code was added to uBlock Origin, it removed the following:
A. Suggested shorts
B. Vertical scroll function during full-screen
C. Horizontal scroll bar during full-screen viewing
Likely due to the addition of the suggested shorts extending beyond what the available screen-space is is for the 'fullscreen' (I'd argue enlarged cinema mode, allowing you to scroll-down and see other content) making it too far off screen, requiring a scroll-bar to see the 'full' content of the page. With the following script added to my filters in my adblocker, it removed the above functions and allowed me to page through 25 videos without getting a horizontal scrollbar.
"www.youtube.com##ytd-reel-shelf-renderer.ytd-item-section-renderer.style-scope"
In the attached image you can see that the video prior adding the above to the adblocker shows that the video is currently full screened and (if you have the same issue, will notice) that the shorts feed is slightly beyond the field of the 'Watch Later' and that the arrow to go Right to the next suggested short goes beyond the fullscreen video's boundaries. For now, I hope this helps, as all we can do is hope this is fixed on their end and not required locally for the foreseeable future.
I'm aware that others have been having this issue for a months as well as I but I cannot find any information specifying or Identifying the Reel Shelf/Shorts section as above. So I apologise if someone else has already stated a fix to this issue, I have been unsuccessful at finding one that works long-term.
