Here’s a feature we bet you haven’t spotted if you’ve upgraded to macOS Sierra.
It’s pretty simple too. If you drag one window so that it butts up against another, there will be an initial period of stickiness during which the window will stick to the other, before overlapping it as happened in earlier releases of OS X.
This makes it really easy to precisely arrange windows. Some people call this feature window snapping, but that’s not quite accurate here because this only works if you drag slowly. If you drag fast then the window overlaps the other just like it’s always done.
Anyway, this is difficult to explain so have a try yourself, and here’s an animated GIF showing it in action.