In macOS Mojave, when in the Displays control panel, on the Arrangement tab, holding down option and dragging a display turns the cursor white. What does this mean? Does it do something special?
MacOS – What’s option-drag in Display Arrangement do
displaymacossystem-prefs
Related Solutions
I have had success with my vertical arrangement by hovering the mouse in the orange areas to bring the dock to the top screen. On the bottom screen you can hover anywhere on the bottom of the monitor.
In your case, where your monitors are almost the same size, it might be a bit tricky to find a spot on the bottom of the top monitor where the cursor wont slip through to the bottom screen and prevent the trigger.
I think I may have just solved it. I have two accounts on this Mac, my Admin account and my daily use account. The admin account didn't have the same screen orientation as the the daily use account, and on the login screen the admin account was listed first. I suspect that when no accounts are logged in that OS X grabs the screen configuration of the first account that it finds - which happened to be the admin account in my case.
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Best Answer
Holding down the Option key allows the currently dragged display to be dropped on top of another display to enable mirroring.
The dragged display still snaps to the edges of the other displays at first, but if you drag further towards the center, the displays can be made to overlap and this turns on the "Mirror Displays" checkbox.