code: function( off, el ) {
  	const ew = getBoundingClientRect( el ).width;
  	off.left = Math.max( 0, Math.min( off.left, viewport.width - ew ) );
  
  	return off;
  }
docs: /*
  Constrain a left to keep the element in the window
  @param  {Object} pl proposed left
  @param  {Number} ew tip element width
  @return {Number}    the best width