„Window snapping on the Mac will snap windows to any of the following targets: edges of other windows, the menu bar, the top of the Dock (if visible), and the sides of the screen.
[…]
While you can't completely turn off window snapping, you can temporarily disable window snapping in macOS with a keystroke action when moving windows around on the screen. To temporarily disable window snapping, hold down the Option (alt) key when you're dragging and moving windows around.“

s.a http://www.maintain.se/blog/window-snapping-in-macos-sierra/