src/offset.js

jQuery.fn.offset = function( options ) {

    if ( arguments.length ) {

        return options === undefined ?

            this :

            this.each(function( i ) {

                jQuery.offset.setOffset( this, options, i );

            });

    }

    var docElem, win,

        elem = this[ 0 ],

        box = { top: 0, left: 0 },

        doc = elem && elem.ownerDocument;

    if ( !doc ) {

        return;

    }

    docElem = doc.documentElement;

    // Make sure it's not a disconnected DOM node

    if ( !jQuery.contains( docElem, elem ) ) {

        return box;

    }

    // If we don't have gBCR, just use 0,0 rather than error

    // BlackBerry 5, iOS 3 (original iPhone)

    if ( typeof elem.getBoundingClientRect !== core_strundefined ) {

        box = elem.getBoundingClientRect();

    }

    win = getWindow( doc );

    return {

        top: box.top + win.pageYOffset - docElem.clientTop,

        left: box.left + win.pageXOffset - docElem.clientLeft

    };

};

jQuery.offset = {

    setOffset: function( elem, options, i ) {

        var curPosition, curLeft, curCSSTop, curTop, curOffset, curCSSLeft, calculatePosition,

            position = jQuery.css( elem, "position" ),

            curElem = jQuery( elem ),

            props = {};

        // Set position first, in-case top/left are set even on static elem

        if ( position === "static" ) {

            elem.style.position = "relative";

        }

        curOffset = curElem.offset();

        curCSSTop = jQuery.css( elem, "top" );

        curCSSLeft = jQuery.css( elem, "left" );

        calculatePosition = ( position === "absolute" || position === "fixed" ) && ( curCSSTop + curCSSLeft ).indexOf("auto") > -1;

        // 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;

        }

        var offsetParent, offset,

            elem = this[ 0 ],

            parentOffset = { top: 0, left: 0 };

        // Fixed elements are offset from window (parentOffset = {top:0, left: 0}, because it is it's only offset parent

        if ( jQuery.css( elem, "position" ) === "fixed" ) {

            // We assume that getBoundingClientRect is available when computed position is fixed

            offset = elem.getBoundingClientRect();

        } else {

            // Get *real* offsetParent

            offsetParent = this.offsetParent();

            // Get correct offsets

            offset = this.offset();

            if ( !jQuery.nodeName( offsetParent[ 0 ], "html" ) ) {

                parentOffset = offsetParent.offset();

            }

            // Add offsetParent borders

            parentOffset.top += jQuery.css( offsetParent[ 0 ], "borderTopWidth", true );

            parentOffset.left += jQuery.css( offsetParent[ 0 ], "borderLeftWidth", true );

        }

        // Subtract parent offsets and element margins

        return {

            top: offset.top - parentOffset.top - jQuery.css( elem, "marginTop", true ),

            left: offset.left - parentOffset.left - jQuery.css( elem, "marginLeft", true )

        };

    },

    offsetParent: function() {

        return this.map(function() {

            var offsetParent = this.offsetParent || docElem;

            while ( offsetParent && ( !jQuery.nodeName( offsetParent, "html" ) && jQuery.css( offsetParent, "position") === "static" ) ) {

                offsetParent = offsetParent.offsetParent;

            }

            return offsetParent || docElem;

        });

    }

});

// Create scrollLeft and scrollTop methods

jQuery.each( {scrollLeft: "pageXOffset", scrollTop: "pageYOffset"}, function( method, prop ) {

    var top = "pageYOffset" === prop;

    jQuery.fn[ method ] = function( val ) {

        return jQuery.access( this, function( elem, method, val ) {

            var win = getWindow( elem );

            if ( val === undefined ) {

                return win ? win[ prop ] : elem[ method ];

            }

            if ( win ) {

                win.scrollTo(

                    !top ? val : window.pageXOffset,

                    top ? val : window.pageYOffset

                );

            } else {

                elem[ method ] = val;

            }

        }, method, val, arguments.length, null );

    };

});

function getWindow( elem ) {

    return jQuery.isWindow( elem ) ? elem : elem.nodeType === 9 && elem.defaultView;

}