I recently joined the Triple Display Crew and have two external monitors hooked up to my MacBook. I ran into a strange problem where the Dock, which I keep down the bottom, wouldn’t pop up on the centre screen.
I wanted my display arrangement to match the real layout as accurately as possible. The external screens are both different sizes with one in portrait orientation, so the layout is a bit weird. The MacBook sits to the left and slightly underneath the centre screen and the other screen sits to the right. Like this:
My guess is that macOS is getting confused since the bottom of the centre screen is not the bottom of the display area in that region, as the MacBook screen is partially underneath it. So to get to the “true bottom” of the centre screen you need to be at the bottom of the left (MacBook internal) screen.
The solution was easy once I guessed this was the problem. Just make the screen layout not quite accurate.
Now the bottom of each screen is distinct and I’ve had no more problems with the Dock popping up. The fact that it doesn’t match reality doesn’t matter too much because the cursor will jump down to the other screen as long as you’re vaguely in the right place and have high enough velocity.
Anyway, I hope that this helps the 0.4% of people that have this particular combination of display layout and Dock position.