Here’s what may be the most subtle new feature of Sierra: when you’re dragging one window next to another, Sierra makes them “stick” slightly when their edges align perfectly. You’re most likely to notice this in the Finder, if you work regularly in two side-by-side windows for moving files around your system, but it applies to any two windows, even those from different apps. To see this effect in action, open two windows and then drag one toward the other, noticing how it stops briefly when the window edges meet up. If the feature gets in the way of aligning windows as you want, press Option to disable it while you’re dragging. This window alignment feature won’t change how you use your Mac, but it should make for a neater screen.