code: function intersection (frame, otherFrame) {
    var x = Math.max(frame.x, otherFrame.x);
    var width = Math.min(frame.x + frame.width, otherFrame.x + otherFrame.width);
    var y = Math.max(frame.y, otherFrame.y);
    var height = Math.min(frame.y + frame.height, otherFrame.y + otherFrame.height);
    if (width >= x && height >= y) {
      return make(x, y, width - x, height - y);
    }
    return null;
  }
docs: Compute the intersection region between 2 frames.
  
  @param {Frame} frame
  @param {Frame} otherFrame
  @return {Frame}