Can't replicate this - testing various zoom levels, the preview is always far enough to the right to not clip.
What browser/version are you using?
Chrome 109.0.5414.120 - One thing to note though is that the preview shows up in the middle of the title which means you need the thread to have a short name but with a big enough content, i do use 125% zoom but it's all the same even if i revert to 100% and just make the window smaller so.. "content" area gets close enough to the sidebar, but it's easier to just go the user preview route since it opens in the same place (and scroll down with it open if it's too far down)
Edit: oh, now that's funny, if you're scrolled all the way up the preview goes on top of it, (and also tested it in Edge, sure it's chromium but it doesn't have any of my extensions)