I have Facebook Connect blocked in Ghostery, so I don't that that is the problem. I also checked the list of blockable items from AdBlockPlus; there's nothing interesting there either. I still experienced three instances today where the post was entered or updated on the site, but my browser did not advance. The last was around 2030 UTC. If I had waited longer, I'm sure I would have gotten the "stay on page?" response.
Any chance this has something to do with the "autosave" facility that applies to postings in this version? I'm going to block that in AdBlock and see if it matters.
After I did that, this post was entered immediately, but who knows what the causation might be? I'll leave autosave disabled for a while and see if that helps.
For the record, I'm blocking: http://ubuntuforums.org/ajax.php?do=autosave