Once you click Reply, the button disappears so that if it takes a little extra time to get processed on the server side, people don't click submit multiple times and ultimately end up double posting accidentally.
It stayed in edit mode. I was able to make the preview window appear/disappear repeatedly. I closed and reopened the page four times, submitting the post each time. The post never appeared. I tried again after submitting this bug report and that time the post went through. If you check it now you will see that only one copy of the post is present. But on the bright side, at least the cursor is in the correct position.