What's Popup Text and Popup Background in the command window properties for?

Press F7 to display a command-line history: you'll see the history appears in a pop-up window, normally magenta on white.

It's just a modal dialog, overlaid on the normal console window, and it disappears when you press Esc, ↵ Enter, or

The key bindings are as follows:
Esc: Cancel the command-line history dialog.
or : Select a command in the list.
↵ Enter: Execute the selected command immediately.
or : Load the selected command on the command line (so that you can edit it).