Move "Thread messages" option from "View" to "Mailbox" menu
Quoting @peterb from #63 (comment 1343687):
The "Thread messages" option is on the "View" menu, but is mailbox-specific (and so perhaps belongs on the "Mailbox" menu, as the other "View" menu options are all global). So when it is not hidden, "... not supported" is popped up in response to a request to thread a particular mailbox.