Build mode: Remove from selection modifier.

[Accensus]

Right now the rectangle selection has only one modifier: add to selection. That means if you mess up and select some blocks by accident, well you're now out of luck and have to start from the beginning or manually deselect each block. That's incredibly tedious.

[FuryoftheStars]

I… want to say it already does. Try holding ctrl while using it. I think it’s an undocumented modifier.

[Accensus]

Nope. I just tried every modifier known to Man. Ctrl only works as deselect with the single block selection tool. Rectangle selection's ctrl acts as "add to selection" and going over selected blocks with it doesn't do anything, i.e. it doesn't work as a "toggle".

[FuryoftheStars]

Huh, yup, you're right.  I could've sworn either it or shift did, but I just fired up the game and tried it, and nope, no dice.  Sorry about that.