[KevinH]

Okay, I stole a monitor from an old linux box and attached it to my iMac dev machine.
Once plugged in, it automatically appear to be situated to the immediate right of my built in screen in my macOS Display setting.

I opened Sigil on Primary and moved it to the secondary monitor and then closed out of Sigil. On next launch of Sigil, the Sigil MainWindow appeared in my second monitor right where it was left.

I closed Sigil and then powered down the second monitor but did not disconnect it and tried launching Sigil. The main screen stayed blank as expected (I could use the Gather Windows button that is part of the Display Settings to see it, or just power up the second monitor).

I next disconnected the second monitor and then tried launching Sigil again, and after a brief delay it appeared on the primary screen having been moved there by Qt automatically.

No crashes, no bugs. It just works on macOS. I will try putting the second monitor on the left in display settings and repeat the tests.

I have enabled some screen geometry debug output and will post a debug build for Windows tonight so we can see why/how Windows is different. Hopefully that may help us figure out what is going on.