jquery/src/offset.js

// The Offset Method
// Originally By Brandon Aaron, part of the Dimension Plugin
// http://jquery.com/plugins/project/dimensions
jQuery.fn.offset = function() {
	var left = 0, top = 0, elem = this[0], results;
	
	if ( elem ) with ( jQuery.browser ) {
		var parent       = elem.parentNode, 
		    offsetChild  = elem,
		    offsetParent = elem.offsetParent, 
		    doc          = elem.ownerDocument,
		    safari2      = safari && parseInt(version) < 522,
		    fixed        = jQuery.css(elem, "position") == "fixed";
	
		// Use getBoundingClientRect if available
		if ( elem.getBoundingClientRect ) {
			var box = elem.getBoundingClientRect();
		
			// Add the document scroll offsets
			add(box.left + Math.max(doc.documentElement.scrollLeft, doc.body.scrollLeft),
				box.top  + Math.max(doc.documentElement.scrollTop,  doc.body.scrollTop));
		
			// IE adds the HTML element's border, by default it is medium which is 2px
			// IE 6 and 7 quirks mode the border width is overwritable by the following css html { border: 0; }
			// IE 7 standards mode, the border is always 2px
			// This border/offset is typically represented by the clientLeft and clientTop properties
			// However, in IE6 and 7 quirks mode the clientLeft and clientTop properties are not updated when overwriting it via CSS
			// Therefore this method will be off by 2px in IE while in quirksmode
			add( -doc.documentElement.clientLeft, -doc.documentElement.clientTop );
	
		// Otherwise loop through the offsetParents and parentNodes
		} else {
		
			// Initial element offsets
			add( elem.offsetLeft, elem.offsetTop );
			
			// Get parent offsets
			while ( offsetParent ) {
				// Add offsetParent offsets
				add( offsetParent.offsetLeft, offsetParent.offsetTop );
			
				// Mozilla and Safari > 2 does not include the border on offset parents
				// However Mozilla adds the border for table or table cells
				if ( mozilla && !/^t(able|d|h)$/i.test(offsetParent.tagName) || safari && !safari2 )
					border( offsetParent );
					
				// Add the document scroll offsets if position is fixed on any offsetParent
				if ( !fixed && jQuery.css(offsetParent, "position") == "fixed" )
					fixed = true;
			
				// Set offsetChild to previous offsetParent unless it is the body element
				offsetChild  = /^body$/i.test(offsetParent.tagName) ? offsetChild : offsetParent;
				// Get next offsetParent
				offsetParent = offsetParent.offsetParent;
			}
		
			// Get parent scroll offsets
			while ( parent && parent.tagName && !/^body|html$/i.test(parent.tagName) ) {
				// Remove parent scroll UNLESS that parent is inline or a table to work around Opera inline/table scrollLeft/Top bug
				if ( !/^inline|table.*$/i.test(jQuery.css(parent, "display")) )
					// Subtract parent scroll offsets
					add( -parent.scrollLeft, -parent.scrollTop );
			
				// Mozilla does not add the border for a parent that has overflow != visible
				if ( mozilla && jQuery.css(parent, "overflow") != "visible" )
					border( parent );
			
				// Get next parent
				parent = parent.parentNode;
			}
		
			// Safari <= 2 doubles body offsets with a fixed position element/offsetParent or absolutely positioned offsetChild
			// Mozilla doubles body offsets with a non-absolutely positioned offsetChild
			if ( (safari2 && (fixed || jQuery.css(offsetChild, "position") == "absolute")) || 
				(mozilla && jQuery.css(offsetChild, "position") != "absolute") )
					add( -doc.body.offsetLeft, -doc.body.offsetTop );
			
			// Add the document scroll offsets if position is fixed
			if ( fixed )
				add(Math.max(doc.documentElement.scrollLeft, doc.body.scrollLeft),
					Math.max(doc.documentElement.scrollTop,  doc.body.scrollTop));
		}

		// Return an object with top and left properties
		results = { top: top, left: left };
	}

	function border(elem) {
		add( jQuery.curCSS(elem, "borderLeftWidth", true), jQuery.curCSS(elem, "borderTopWidth", true) );
	}

	function add(l, t) {
		left += parseInt(l) || 0;
		top += parseInt(t) || 0;
	}

	return results;
};