>Ah, OK, so Michel has already implemented this as a toggle. Right now I see no indication on a message that it's marked, so if Michel could at least change the button label to "Unmark" that would be better, I think.

I agree that changing a label and icon would be better but it works fine for me the way it is. As I mentioned, you can see current status in the tooltip. The Filter button works the same way, btw.