Mail viewer window doesn't appear

This started happening recently on my father's 15" Retina Macbook Pro running El Capitan 10.11.3: when he starts Mail, the main viewer window does not appear.

It's there, but invisible: I know this because selecting "Minimize" from the [Windows] menu in mail makes it appear for a fraction of a second before it zooms down to the Dock. Once there, you can click on the Dock icon and it will zoom back onto the screen, after which things operate normally.

Until you reboot the computer, or quit and start Mail again.

Curiously enough, this problem doesn't occur when I boot the machine in Safe Mode. However, I've disabled all login items and the problem still occurs with a normal boot.

I've never seen anything like this and would be grateful for any hints or suggestions...

Has invited:

howapple - hello

Favor from:

Do you have more than one space in Mission Control?

OS X El Capitan: See open windows and spaces in Mission Control

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