I'm having a very strange problem with Mail on a new iMac which is running 10.10.3. The Apple store migrated the user's home files over from an old MacBook, which was running Snow Leopard, and everything worked fine for a few weeks. Then the problem appeared: Mail wouldn't show any windows when starting. The program was clearly running; the menu bar showed "Mail" and a dot appeared under Mail's Dock icon. Also, Mail's "window" menu had entries for "Inbox" and two message windows which were open the last time Mail was shut down. The weird thing was that choosing "Enter Full Screen" from the View menu actually worked; the Inbox window would appear full-screen, and I could then choose "Exit Full Screen" and the Inbox window would appear normally. Quitting and restarting Mail made the problem happen again, though - no window appeared.
Apple's chat and phone support have had me deleting plist and lock files from various locations, but no joy. They have actually talked the user through two complete reinstallations of Yosemite, which took 24 hours each time as her net connection is very slow. They have said something very odd, which I doubt is correct: they think the problem with Mail may have been caused by me copying files over from another Mac. Here's what I did: I made a subdirectory in her Documents folder and copied everything from the home directory of another user on a Mac Mini to that subdirectory. I did this to preserve those files as the Mini is old and the user wants to get rid of it.
I don't see how copying files - no matter what files - to a subdirectory in Documents could possibly have broken Mail's ability to display its window when starting up. Mail is fixed now, after two reinstallations of Yosemite, but I haven't copied the files over again. Apple's support guy is recommending that she buy a new backup drive and copy the files to that instead. I don't like this for two reasons: one, it means spending more money, and two, the files won't be backed up as they would be if we kept them on her iMac.
Can you think of anything which could have caused Mail to stop showing its window only when first started, or any way in which copying files over could have caused it? Can you offer any insight into this weird problem?
Update: After a trip to the Apple store where the staff reinstalled Yosemite yet again, Mail is working normally. But here's the kicker: a few weeks later, Safari started doing exactly the same thing - no window appears when you run the program, but entering full screen makes it appear, and exiting full screen then works properly. Does anyone have any ideas?