I don't have an iPad.
We've got 2 iPhone7+'s but have not set them up as alternative/additional displays.
(I don't even know how to do that!)
But I recently got a "USB-C Digital AV Multi-port Adapter, AVOTCH USB C Hub ，
3.1 Type C Hub , HDMI VGA Dual screen display Output, Card Reader, 2 USB
3.0 Ports and Gigabit Ethernet Adapter"
And it has HDMI and VGA outputs.
Your reference to "Gather Windows" showed up in my System Preferences > Display.
I couldn't figure it out but with your (most erudite and astute!) discussion I clicked to
"Gather Windows" and could now understand that what I'd previously seen and not
understood was actually another display referenced to as the result of that multi adapter.
All I did was unplug the multi adapter from the USB-C (charging) port of my MacBook
and the 2nd display disappeared!
Kind of odd that even with no actual HDMI or VGA device plugged into that multi
adapter, the MacBook still read it as though there was (one or the other) attached
to the MacBook.
So ... thanks!
Now the mouse doesn't slide off the screen anymore!