If you scrollback in either the mission or the chat tab, then switch to the other tab, the red background from the scrolledback tab appears in the middle of your screen. Rather annoying.

I agree, this has been around for awhile, happens on mac here too.

This is done on purpose so we don't get bug reports about the info not updating because they have the info unknowningly scrolled back.

Except, if you scroll back on the mission tab, and switch to the general tab, the general tab should update, right? So why does it need the red background?

The red would only be where the scrolled back window is. In your example, the red would be where the mission chat goes, which is in the (sortof) middle of the screen.

The scrolled back text wouldn'e update, so in your example, the mission text wouldn't update and new players would get confused.
The general tab would also update and not have the red background.