I just wrote an answer. When previewed before posted, everything was fine. However when posted, a long formula got truncated. So it seems the preview box is slightly wider than answer box
Another more serious issue is sometimes when previewing, somehow the preview window strangely changes (and is slightly shifted if it's not my delusion), and preview button disappears, so I can't even cancel the preview. (I think this happened when "preview" was clicked, and I was editing in the source mode.)