If you drag a window's corner, the adjoining edges resize. If you hold down Option while you do this, all four sizes resize about the center. Even better, you c

If you drag a window's corner, the adjoining edges resize. If you hold down Option while you do this, all four sizes resize about the center. Even better, you can press and release Option during a single drag to turn this behavior on and off incrementally, which means you can arbitrarily adjust a window with one drag (if you're persistent enough).