Thanks for the test. The message did display on my Special:Newmessages page. I marked it as "read", but afterwards the watchlist page still says there are newmessages.
It used to be that my messages to others would show up on the Special:Newmessages page. It is these messages that I wrote to others that I am thinking are causing the the button to activate.
When I go to the Special:Newmessages page using the button on my Watchlist page it says only:
- New messages
- [Mark all as read] button
- Messages sent to you:
- Messages on other talk pages:
I'm glad to know that messages from others may show up.
As you suggest, it may be an LQT problem. I'm glad to hear that there's an effort being planned to address this software.
Thanks for looking into the problem.