Apple mail inbox disappears every time I tab back to it

I have two monitors and I hate the new Mail. Mostly because I can't open a window and drag it to the other screen. But my big problem is that I use command tab to move from app to app and every time I tab back to mail, nothing is visible. I have to go to window: inbox. Then I can see the inbox again. Also, it's the only program that when I am on it, the left hand screen goes black. I took "spaces" off, but there must be some other pref on that I don't know about. OS 10.10.2. Mail 8.2 on Mac Pro.