The issue is caused by the scrollbar appearing and disappearing. That text field is not supposed to have a scrollbar, since it resizes itself to match the text inside.
I couldn't reproduce it exactly, but at some zoom levels, I managed to get a scrollbar to appear consistently (although without the annoying effect where it flickers).
By the way, on my Debian (“stretch”) using Firefox ESR, I pretty consistently get a scrollbar for every second line (i.e. if no scrollbar appears for an n lines long draft, it appears when the draft becomes n+1 lines long, disappears when it’s n+2 lines long, reappears when it’s n+3 lines long, and so on). Probably this was the case since the very beginning, I was just too lazy to report it… Thanks for reporting and fixing!