Using "Next unread message" not quite as expected

Hi!

Another observation with the latest version pulled from github: Suppose  
there's an mbox with several unread messages. You open one message by  
double-clicking it in the main window. This will bring up a new window  
for just this message. The new window contains a button "Next unread".

Expected behavior: The next unread message from that mbox will be  
displayed in the same window where you've clicked the button.
Actual behavior: The new window still is open and displays the first  
message. The next unread message will be displayed, but in the main  
window.

'That's particularly bad if the main window is on a different virtual  
desktop or hidden behind other windows: You won't notice that new  
message is displayed there, so you may hit the "Next unread" button  
repeatedly without (apparently) anything happening, while one message  
after the other is marked as read in the hidden window. If you don't  
realize what's happening, you *could* miss important mails.

It would be great if you could look into that as well. (Perhaps I just  
didn't see a setting controlling that behavior, in that case I  
apologize for the noise. :-)

Best regards

Andreas