windows open on secondary monitor

in Genius Bar
Opening a window sends it to the secondary monitor instead of the main monitor even when the secondary is  turned off.  In order to retrieve the window, it's necessary to turn on the secondary monitor and manually move the window to the main monitor.  

I've tried many suggestions after googling the problem. So far, I haven't found an answer that works. Opening display and using "Gather Windows' works for parts of the Display Window itself but not for other windows.

Mirroring the monitors helps but then presents another problem when I want to use the secondary monitor for another purpose. Turning mirroring on and off is a pain. How can the problem be remedied?

This problem seemed to have started when I upgraded to Mojave., although I couldn't swear that Mojave caused it.

Any suggestions to make windows open on the main monitor?

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