Try to start Steam like this: STEAM_FRAME_FORCE_CLOSE=1 steam
As far as I can see it's deactivated because the tray doesn't work for everybody. There is no way to maximize Steam again if the tray isn't working.
This is a known issue and related to an issue with app indicator support.
We deliberately switched the close box to minimize because of the number of people where the app indicator didn't work. If the window actually closes and there is no app indicator there's no UI at all for Steam even though it's still running, so we switched to minimizing. Unfortunately there's no way to know if app indicators are working (presence/absence is not sufficient as even if they're present they do not always work). Our hope is that the problems there will get worked out and then we can remove our workarounds and rely on the app indicator, at which point closing the window will close it.
Maybe a good alternative to avoid user confusion is to DISABLE/grey-out the close button? (Perhaps even with a hover popup explaining why?)
perhaps a sensible workaround is to have an option to hide the main window in the tray menu, that way itll only be usable if the indicator is truly working
Nothing extracted yet.
Both the minimise and close buttons perform the same task, minimising the window. There is effectively no close button as a result.
System Info: http://pastebin.com/JGrFkdeX