Go to a task → start writing new comment → write long enough comment that you have scrollbar in the text area → then scroll up and select 2-3 words in upper part of the textarea → click on link action in the toolbar → the comment scrolls to bottom making whatever you are doing disappear due to scroll
Sounds like a very specific path to reproduce but some of us are hitting it regularly.
This problem happens in Chrome, not in Firefox.
Wikimedia report: https://phabricator.wikimedia.org/T78027