This is not something we are considering because it would cause a cascade event.
Say for example you have a bunch of messages in unread, and you click on one of them. After a few seconds it is marked as read and removed from the list. The next one is automatically selected as you step away from the computer for a minute. In your absence, the next one is marled as read an removed, then the next one and the next one etc. When you return with your coffee, there are no longer any unread messages.
So we leave the read messages in the Unread folder until you move away from that folder. The next time you come back to the Unread folder, only unread messages will be there.