Currently, the only way to deselect is with Shift + Select, which is also the box-shape select. So there isn’t actually a way to Paint to Deselect and using the box-shape selector isn’t as ‘easy’ to sometimes get exactly what I want unselected.
This is related to my question discussing the new Box feature: Discord where I mentioned maybe CTRL + Click could be a way to move the box without moving the cards within, maybe CTRL + Click (for painting) could be unselect with the painter.