A couple of things…
Might have been said before: it would be really nice if we could have a parameter to move the mask in from the edges, rather than geometric resize.
and… sometimes, when adding, a previously-selected area gets dropped. Sometimes, when adding, nothing gets added but the previously-selected area shrinks. Is there an instance where a user would want “add” to mean “remove” or “shrink?” I don’t think so.