If you’re watching a video in picture-in-picture mode in macOS Sierra, it snaps to the nearest corner. Even when you drag it out of that corner, it will again snap to the corner nearest to where you let it go. You can use command-drag to put the video where you want it.
Default macOS Sierra Picture-in-Picture Behavior
I used our Method 2 for entering macOS Sierra Picture-in-Picture mode (there’s also Method 1) while watching the new Rogue One trailer. It snapped to the upper right corner. This behavior makes sense because if you’re using PiP mode in the first place, you probably want the video out of the way. Note that I enlarged the default size of the video for this screenshot.
The next part makes less sense, because if I drag the video to another part of the screen, it snaps to the nearest corner, as shown below.
Frankly, I expect my windows to stay where I put them, but Tim Cook didn’t ask me. But, if you Command-Drag the video, it stays where you leave it, as shown below. “Command-Drag” means simply holding down the Command key while you’re dragging the window around.
If you move it again, it will once again snap to a corner unless you use Command-Drag.