Hello!
I just experienced the same issue due to server-side sorting of mails (in my case, using procmail). This is quite annoying because I keep an eye on the browser tab for new mails while I’m working. But when there’s a new mail incoming and presorted in another folder, Roundcube does not show e.g. “(1) Roundcube”. I can only check it by manually activating the browser tab for a look.
So all in all, it would be nice to have all unread mails over all folders summarized in title bar, instead of just the number of unread mails in inbox.
Best is a configuration option, but if this is too much to code, a patch for this function would be great, too.