When submitting a post, it normally adds your reply to the page, and highlights it for 5 seconds. In this particular case, the highlight added several hundred pixels of padding to the bottom of the page, and highlighted in the wrong place. Best explained with a screenshot:
Here's the diff:
https://en.wikipedia.org/w/index.php?title=Talk:Sun&diff=1012503877&oldid=1012503422&diffmode=source
Also, if you'd prefer I place these bug reports somewhere else such as Phabricator or GitHub, let me know. Thanks.