Close button sends client to the tray, statically linked Qt for OSX.
See the Changelog for list of features and fixes currently only available in the nightly builds.
The problem with that is that due to memory considerations the server doesn't keep a persistent record of who's on whose list, that's something the client informs the server about when they log in. Although to be honest I'd have to think real hard about whether I wanted to make this change even if it were practical. There's a variety of contexts in which you might add someone to your list, and not all of them are positive.