Normally, double-clicking a separator bar expands that editor to be fully maximized (collapsing the other editors). That's still true, but now, if you double-click that same separator bar after expanding it will return the editor the previous state before expansion. Now it's easier to temporarily maximize an editor and then jump back to how it was before.
— Dave DeSandro (@desandro) November 6, 2015
If you double-click any other editor separator bar (not the one you just maximized), it will maximize that one.