Dimensions: allow modification of coordinates argument

Ref f7e60dc83d81cbf892de9dab39642dd67c49bd23

src/offset.js

@@ -57,7 +57,9 @@ jQuery.offset = {
 		}
 
 		if ( jQuery.isFunction( options ) ) {
-			options = options.call( elem, i, curOffset );
+
+			// Use jQuery.extend here to allow modification of coordinates argument (gh-1848)
+			options = options.call( elem, i, jQuery.extend( {}, curOffset ) );
 		}
 
 		if ( options.top != null ) {

test/unit/dimensions.js

@@ -470,4 +470,16 @@ testIframe( "dimensions/documentLarge", "window vs. large document", function( j
 	ok( jQuery( document ).width() > jQuery( window ).width(), "document width is larger than window width" );
 });
 
+test( "allow modification of coordinates argument (gh-1848)", 1, function() {
+	var element = jQuery( "<div/>" ).appendTo( "#qunit-fixture" );
+
+	element.offset(function( index, coords ) {
+		coords.top = 100;
+
+		return coords;
+	});
+
+	equal( element.offset().top, 100, "coordinates are modified" );
+});
+
 })();