My IMAP account has two folders and I would like to make one a sub-folder of the other. If I click and drag folder B onto folder A I get the message: The current command did not succeed. Tha mail server responded: RENAME failed: Mailbox B is in use by another process. Now it seems to me that because I click on (and then drag) folder B, this click selects folder B and it's contents are shown in another pane. This is what marks the folder as busy! I think that solution should be that folders should only be opened / selected / "made budy" upon mouse-up and not mouse-down. This is with Mozilla/5.0 (X11; U; Linux 2.2.16-22 i686; en-US; 0.8.1) Gecko/20010326.
I believe this is fixed in the 0.9.1 Marking WORKSFORME.
Verified worksforme on Linux 06-18-08-trunk build