What's this plastic cover under the touchscreen? Can you pluck it out and stick something more useful there, like a cup holder?

That's the IR sensor that detects the presence of your hand and causes the pop-up menu to appear on the screen.

The what?

Pir (passive infra red) or ir (infra red) in this case. Similar to some burglar and door entry systems.

Edited 26 December, 20178 yr by Danny 57

No, I mean what "popup menu"?

It is a proximity  sensor that brings up the on screen menus when your fingers go near the screen.