User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0 Build ID: 20180726100339 Steps to reproduce: (Please refer to the video attached.) 1. In https://bugzilla.mozilla.org/userprefs.cgi?tab=settings > User Interface, set "Expand the comment box dynamically" to on. Save the change. 2. In the comment box of a bug report, type a long paragraph so the comment box is expanded. 3. Go back a few lines and try to insert characters there. Actual results: 1.The page bounces every time I type a character. You may need to scroll the page/comment box a few times before typing to trigger the issue. 2. When typing at the last line, the comment box often snaps to the bottom of browser window. It should not scroll the buttons below comment box out of the view. Note: It can not be reproduced if "Expand the comment box dynamically" is set to off (site default).

I have seen this once.