This addresses a complaint/question that comes up every now and then, and would probably provide a lot of side-benefits that we can't even think of at the moment. It would be excellent if mutt could view the contents of several folders at the same time, in an integrated display (i.e. there's not necessarily any visible distinction between messages in one folder and another).
If this worked, one could (for example) set limits and search within multiple folders.
It MAY even be useful to add another % option to index_format that allows each message to display which folder it's from, and a new sorting option to sort (or sort_aux) by folder.
Here's the way I imagine the interface could work (just a suggestion). <change-folder> works as it currently does (namely, it clears the index and rebuilds the index from the contents of the new folder), however we can add a new command: <add-folder>. This command would behave similar to <change-folder> (i.e. the prompt, the cycling through mailboxes, etc.) but instead of clearing the index and rebuilding it with the contents of the new folder, it adds the contents of the new folder to the existing index list, and re-sorts the index to merge the two together.
If this could work generically, one could theoretically view both the local mailspool and an IMAP INBOX at the same time (for example). You could also view all new mail messages at the same time in all of your mailboxes (by adding them all, and then limiting the view to just new messages). You could even search all mailboxes by adding all mailboxes and then searching.
Thinking beyond this, really blue-sky, would be if mailboxes were handled similar to the way threading is handled: that all messages in a given mailbox could be "collapsed" the way a thread can be collapsed.
I don't know how difficult any of this would be to implement. It would be awesome, though: very mutt-like, very simple interface, and extraordinarily powerful (better than any other mail reader out there, bar none).
