Mail viewer window keeps disappearing. (old question, asking again)

system: MacBook Pro, 13", Mid 2014

Dual monitor: MAC LED Cinema Display, 1920 x 1200 (WUXGA - Widescreen Ultra eXtended Graphics Array)

Mail Version 11.3 (3445.6.18)

OS: High Sierra 10.13.4

Issue: Mail viewer window disappears immediately upon opening, as if it's going to another desktop, but it does not appear on any desktop. The same thing happens to emails that are opened from the ribbon -- they appear for an instant then disappear.

Workarounds:

1. A 2016 post suggested opening the OS in recovery mode, then reopening it as per normal.
2. In my case, quitting and re-opening mail mail generally fixes the "hidden window" problem.

   Sometimes Force Quit is required. The cause appears to be re-opening a draft which immediately vanishes. When you try to quit Mail, there's a quiet beep, and nothing happens. I suspect Mail is asking whether you want to save or delete the draft. Since you can't access the draft (because it's invisible), you can't answer the dialog, so you can't quit Mail.

This is a bug. We shouldn't need to use work-arounds like doing a hard-shutdown because mail won't quit or Force Quitting Mail, or quitting and re-opening Mail.