Is your feature request related to a problem? Please describe.
I use simple area selection mode and use alt+scroll to set the positions of the area selection corners. Often, I set my first corner and then set my second corner and want to modify the location of my first corner.

Describe the solution you'd like
I would love it if there were a hotkey to allow me to toggle between the active selection corner so that I can toggle back and forth between them and use alt+scroll to set their positions.

This is now on my todo list. I kind of thought it would have been there already, but I couldn't find it.

Just to make sure you are aware of the current functionality as well: You can middle click with the tool to select the corners, assuming you are close enough to be able to see them/point at them. (the ray trace max range is something around 200 blocks) You can also select the active/selected corner via the checkmarks in the Area Editor menu. And also are you using the Corners or the Expand selection mode? In most cases at least I prefer the Expand mode. And at least in creative worlds with technical builds floating in the air the growSelection hotkey is really handy. But I still definitely need to also add a way to nudge adjust a given side of the selection box, rather than relying on moving a particular corner of the selection.