Picture-in-picture is one of the best new features in macOS Sierra, but when you try to move the video around your screen it automatically snaps to the closest corner. OS X Daily points out you can move it where you want by holding down the Command key.
After you create a picture-in-picture window, just hold down the Command key then click on the video and move it wherever you want on your screen. The video will stay where you leave it and won't snap to the corner.
Move a PIP Video Player Window Anywhere on Mac [OS X Daily]