Currently, you need to specify the room ID/Name to use /leave. "/leave" should be enough, but keeping the ability to optionally include a room ID.
This should already possible, does it not work for you?
OK I re-tested this and it is possible but you need to hit enter twice.
Well yes, that seems intentional to me - once to confirm the command suggestion and another to actually do it.
This seems a bit weird to me, you can leave with one enter if you specify the name.