Can you provide more reproduction steps? When we try to reproduce this, the dropdown does go away.
Potential Bug in Forum (Private) Messaging
I had observed a little weird experience with one of the forums where a spammer messaged me privately. And the room that was created shows me first as entered.
This is actually counter-intuitive. I had reported the same in that forum and the developers have confirmed that the forum engine (NodeBB) seems to be having this limitation.
I would like to bring the same for the kind attention of NodeBB team so that they can assess the feasibility of addressing the same.