jquery/src/offset.js

var rroot = /^(?:body|html)$/i;

jQuery.fn.offset = function( options ) {
	if ( arguments.length ) {
		return options === undefined ?
			this :
			this.each(function( i ) {
				jQuery.offset.setOffset( this, options, i );
			});
	}

	var box, docElem, body, win, clientTop, clientLeft, scrollTop, scrollLeft, top, left,
		elem = this[ 0 ],
		doc = elem && elem.ownerDocument;

	if ( !doc ) {
		return null;
	}

	if ( (body = doc.body) === elem ) {
		return jQuery.offset.bodyOffset( elem );
	}

	docElem = doc.documentElement;

	// Make sure we're not dealing with a disconnected DOM node
	if ( !jQuery.contains( docElem, elem ) ) {
		return { top: 0, left: 0 };
	}

	box = elem.getBoundingClientRect();
	win = getWindow( doc );
	clientTop  = docElem.clientTop  || body.clientTop  || 0;
	clientLeft = docElem.clientLeft || body.clientLeft || 0;
	scrollTop  = win.pageYOffset || docElem.scrollTop;
	scrollLeft = win.pageXOffset || docElem.scrollLeft;
	top  = box.top  + scrollTop  - clientTop;
	left = box.left + scrollLeft - clientLeft;

	return { top: top, left: left };
};

jQuery.offset = {

	bodyOffset: function( body ) {
		var top = body.offsetTop,
			left = body.offsetLeft;

		if ( jQuery.support.doesNotIncludeMarginInBodyOffset ) {
			top  += parseFloat( jQuery.css(body, "marginTop") ) || 0;
			left += parseFloat( jQuery.css(body, "marginLeft") ) || 0;
		}

		return { top: top, left: left };
	},

	setOffset: function( elem, options, i ) {
		var position = jQuery.css( elem, "position" );

		// set position first, in-case top/left are set even on static elem
		if ( position === "static" ) {
			elem.style.position = "relative";
		}

		var curElem = jQuery( elem ),
			curOffset = curElem.offset(),
			curCSSTop = jQuery.css( elem, "top" ),
			curCSSLeft = jQuery.css( elem, "left" ),
			calculatePosition = ( position === "absolute" || position === "fixed" ) && jQuery.inArray("auto", [curCSSTop, curCSSLeft]) > -1,
			props = {}, curPosition = {}, curTop, curLeft;

		// need to be able to calculate position if either top or left is auto and position is either absolute or fixed
		if ( calculatePosition ) {
			curPosition = curElem.position();
			curTop = curPosition.top;
			curLeft = curPosition.left;
		} else {
			curTop = parseFloat( curCSSTop ) || 0;
			curLeft = parseFloat( curCSSLeft ) || 0;
		}

		if ( jQuery.isFunction( options ) ) {
			options = options.call( elem, i, curOffset );
		}

		if ( options.top != null ) {
			props.top = ( options.top - curOffset.top ) + curTop;
		}
		if ( options.left != null ) {
			props.left = ( options.left - curOffset.left ) + curLeft;
		}

		if ( "using" in options ) {
			options.using.call( elem, props );
		} else {
			curElem.css( props );
		}
	}
};


jQuery.fn.extend({

	position: function() {
		if ( !this[0] ) {
			return null;
		}

		var elem = this[0],

		// Get *real* offsetParent
		offsetParent = this.offsetParent(),

		// Get correct offsets
		offset       = this.offset(),
		parentOffset = rroot.test(offsetParent[0].nodeName) ? { top: 0, left: 0 } : offsetParent.offset();

		// Subtract element margins
		// note: when an element has margin: auto the offsetLeft and marginLeft
		// are the same in Safari causing offset.left to incorrectly be 0
		offset.top  -= parseFloat( jQuery.css(elem, "marginTop") ) || 0;
		offset.left -= parseFloat( jQuery.css(elem, "marginLeft") ) || 0;

		// Add offsetParent borders
		parentOffset.top  += parseFloat( jQuery.css(offsetParent[0], "borderTopWidth") ) || 0;
		parentOffset.left += parseFloat( jQuery.css(offsetParent[0], "borderLeftWidth") ) || 0;

		// Subtract the two offsets
		return {
			top:  offset.top  - parentOffset.top,
			left: offset.left - parentOffset.left
		};
	},

	offsetParent: function() {
		return this.map(function() {
			var offsetParent = this.offsetParent || document.body;
			while ( offsetParent && (!rroot.test(offsetParent.nodeName) && jQuery.css(offsetParent, "position") === "static") ) {
				offsetParent = offsetParent.offsetParent;
			}
			return offsetParent;
		});
	}
});


// Create scrollLeft and scrollTop methods
jQuery.each( {scrollLeft: "pageXOffset", scrollTop: "pageYOffset"}, function( method, prop ) {
	var top = /Y/.test( prop );

	jQuery.fn[ method ] = function( val ) {
		return jQuery.access( this, function( elem, method, val ) {
			var win = getWindow( elem );

			if ( val === undefined ) {
				return win ? (prop in win) ? win[ prop ] :
					win.document.documentElement[ method ] :
					elem[ method ];
			}

			if ( win ) {
				win.scrollTo(
					!top ? val : jQuery( win ).scrollLeft(),
					 top ? val : jQuery( win ).scrollTop()
				);

			} else {
				elem[ method ] = val;
			}
		}, method, val, arguments.length, null );
	};
});

function getWindow( elem ) {
	return jQuery.isWindow( elem ) ?
		elem :
		elem.nodeType === 9 ?
			elem.defaultView || elem.parentWindow :
			false;
}