Command-Drag the Picture-in-Picture Window Anywhere on Your Mac
Picture-in-picture is one of the best new features in macOS Sierra , but when you try to move a video around the screen, it automatically switches to the closest corner. OS X Daily indicates that you can move it wherever you want by holding down the Command key.
After creating the picture-in-picture window, simply hold down the Command key, then click the video and move it anywhere on the screen. The video will stay where you leave it and won’t end up in a corner.
Move PIP Video Player Window Anywhere on Mac | OS X Daily