The title is a bit vague, but I'll try to elaborate. Messages in the message inbox have two icons, unread and read. Any message is considered. If the player clicks on a message to view it, it receives focus, the message is considered read, and the icon changes. In some cases when the message receives focus it retain the icon for unread messages.
1. Start a new game. You will automatically receive two messages; HQ added, and game started.
2. Hover the cursor over the message button (to the far right at the bottom of the screen). It will display "2 new messages".
3. Click to open the inbox. The first message will be selected, though it will display the unread icon.
4. Move your mouse cursor slightly. The tooltip on the message button will display "1 new message".
5. At this point there are two different ways to select the other message:
A. You click on the other message. Both the message selected, and the one who was previously selected will change icons to read. The messages button will show the icon of a read message, and the tooltip text "Messages". (This is expected, as all messages have been selected and viewed/read.)
B. You press 'del' to archive the selected message. The message will be moved to the archive, and the remaining message will be selected. However, while the message button behaves as in A, the message itself shows the icon of an unread message.
If the message window is closed and reopened, the correct icon will be shown. Also if the cursor hovers over any of the buttons or other controls in the inbox window, the icon changes.
I think the a message change icon to read once it is receives focus. This would make the information consistent with the information offered by the messages button, as the inbox in step 3 would display one read and one unread message, and the inbox in B would show one read message.
Widelands bzr r5784 on Ubuntu 10.04
Fixed in revision 6033.