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;
}

src/offset.js

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