Trying to drag the content horizontally causes it to attempt to scroll then immediately rebound. On its own this is a strange stylistic decision, but coupled with the sidebar position being fixed it just looks sloppy.
This isn't mobile only, on desktop the center content can be scrolled horizontally and it overlaps very oddly with the side content there too.
Maybe this is an iOS thing because of it's "rubber band" scrolling. In any case this is just Mastodon's standard UI, so maybe you could open a bug over there?
This isn't mobile only, on desktop the center content can be scrolled horizontally and it overlaps very oddly with the side content there too.
All in all it feels like a rushed job.