What's option-drag in Display Arrangement do? 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?
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.