I just upgraded from v7 to v8.1.876 and I noticed a small flaw. Go to: Settings => Mail => Replies and Forwards => Reply Header / Forward Header => Style => Custom. In the Preview window I can’t scroll down to edit the last lines. I have to paste it from the Clipboard. It’s no big issue, but it should be corrected.
I am seeing a scroll bar, and can also scroll with the mouse wheel when hovering over the field. When I am not hovering, there is no scrollbar visible.
Are you maybe using a custom Theme? If you are change back to the default Modern Theme and see if there is any difference. If you want wider scroll bars, you can change them in the Theme editor, or just change to the Classic Theme.