Long comments lose "post comment" button

In the process of troubleshooting a problem with Twenty Sixteen and Jetpack comments at hot-topic.co.nz, I have discovered that if you try to post a sufficiently long comment (copy and pasted text works, as in this example), the “posting as” line scrolls off the bottom of the page, and doesn’t return if you place the cursor at the bottom of the text box. This makes posting the comment impossible…