Hello @chrylis, are you still experiencing this issue on an up to date system?
Back in 2015, this was the intended behavior for the close button and the default behavior controlled by STEAM_FRAME_FORCE_CLOSE has changed since then.
Nothing extracted yet.
After the latest update (Jul 3 2015, 13:50:32), clicking the "close" button on the main client screen fails to close the client window but only minimizes it to the taskbar. (And the longstanding bug where sending the X close command via keyboard or selecting "close" from the taskbar exits Steam entirely is still in place.)