Landing pull request 477. 1.7 jQuery.offset.supportsFixedPosition. Fixes #6809.

More Details:
 - https://github.com/jquery/jquery/pull/477
 - http://bugs.jquery.com/ticket/6809

src/offset.js

@@ -63,8 +63,6 @@ if ( "getBoundingClientRect" in document.documentElement ) {
 			return jQuery.offset.bodyOffset( elem );
 		}
 
-		jQuery.offset.initialize();
-
 		var computedStyle,
 			offsetParent = elem.offsetParent,
 			prevOffsetParent = elem,
@@ -120,46 +118,22 @@ if ( "getBoundingClientRect" in document.documentElement ) {
 	};
 }
 
-jQuery.offset = {
-	initialize: function() {
-		var body = document.body, container = document.createElement("div"), innerDiv, checkDiv, table, td, bodyMarginTop = parseFloat( jQuery.css(body, "marginTop") ) || 0,
-			html = "<div style='position:absolute;top:0;left:0;margin:0;border:5px solid #000;padding:0;width:1px;height:1px;'><div></div></div><table style='position:absolute;top:0;left:0;margin:0;border:5px solid #000;padding:0;width:1px;height:1px;' cellpadding='0' cellspacing='0'><tr><td></td></tr></table>";
-
-		jQuery.extend( container.style, { position: "absolute", top: 0, left: 0, margin: 0, border: 0, width: "1px", height: "1px", visibility: "hidden" } );
-
-		container.innerHTML = html;
-		body.insertBefore( container, body.firstChild );
-		innerDiv = container.firstChild;
-		checkDiv = innerDiv.firstChild;
-		td = innerDiv.nextSibling.firstChild.firstChild;
-
-		this.doesNotAddBorder = (checkDiv.offsetTop !== 5);
-		this.doesAddBorderForTableAndCells = (td.offsetTop === 5);
-
-		checkDiv.style.position = "fixed";
-		checkDiv.style.top = "20px";
-
-		// safari subtracts parent border width here which is 5px
-		this.supportsFixedPosition = (checkDiv.offsetTop === 20 || checkDiv.offsetTop === 15);
-		checkDiv.style.position = checkDiv.style.top = "";
+jQuery.offset = {};
 
-		innerDiv.style.overflow = "hidden";
-		innerDiv.style.position = "relative";
+jQuery.each(
+	( "doesAddBorderForTableAndCells doesNotAddBorder " +
+		"doesNotIncludeMarginInBodyOffset subtractsBorderForOverflowNotVisible " +
+		"supportsFixedPosition" ).split(" "), function( i, prop ) {
 
-		this.subtractsBorderForOverflowNotVisible = (checkDiv.offsetTop === -5);
-
-		this.doesNotIncludeMarginInBodyOffset = (body.offsetTop !== bodyMarginTop);
+	jQuery.offset[ prop ] = jQuery.support[ prop ];
+});
 
-		body.removeChild( container );
-		jQuery.offset.initialize = jQuery.noop;
-	},
+jQuery.extend( jQuery.offset, {
 
 	bodyOffset: function( body ) {
 		var top = body.offsetTop,
 			left = body.offsetLeft;
 
-		jQuery.offset.initialize();
-
 		if ( jQuery.offset.doesNotIncludeMarginInBodyOffset ) {
 			top  += parseFloat( jQuery.css(body, "marginTop") ) || 0;
 			left += parseFloat( jQuery.css(body, "marginLeft") ) || 0;
@@ -210,10 +184,11 @@ jQuery.offset = {
 			curElem.css( props );
 		}
 	}
-};
+});
 
 
 jQuery.fn.extend({
+
 	position: function() {
 		if ( !this[0] ) {
 			return null;

src/support.js

@@ -20,7 +20,8 @@ jQuery.support = (function() {
 		events,
 		eventName,
 		i,
-		isSupported;
+		isSupported,
+		offsetSupport;
 
 	// Preliminary tests
 	div.setAttribute("className", "t");
@@ -249,6 +250,49 @@ jQuery.support = (function() {
 		}
 	}
 
+	// Determine fixed-position support early
+	offsetSupport = (function( body, container ) {
+
+		var outer, inner, table, td, supports,
+			bodyMarginTop = parseFloat( body.style.marginTop ) || 0,
+			ptlm = "position:absolute;top:0;left:0;width:1px;height:1px;",
+			style = "style='" + ptlm + "margin:0;border:5px solid #000;padding:0;'",
+			html = "<div " + style + "><div></div></div>" +
+							"<table " + style + " cellpadding='0' cellspacing='0'>" +
+							"<tr><td></td></tr></table>";
+
+		container.style.cssText = ptlm + "border:0;visibility:hidden";
+
+		container.innerHTML = html;
+		body.insertBefore( container, body.firstChild );
+		outer = container.firstChild;
+		inner = outer.firstChild;
+		td = outer.nextSibling.firstChild.firstChild;
+
+		supports = {
+			doesNotAddBorder: (inner.offsetTop !== 5),
+			doesAddBorderForTableAndCells: (td.offsetTop === 5)
+		}
+
+		inner.style.position = "fixed";
+		inner.style.top = "20px";
+
+		// safari subtracts parent border width here which is 5px
+		supports.supportsFixedPosition = (inner.offsetTop === 20 || inner.offsetTop === 15);
+		inner.style.position = inner.style.top = "";
+
+		outer.style.overflow = "hidden";
+		outer.style.position = "relative";
+
+		supports.subtractsBorderForOverflowNotVisible = (inner.offsetTop === -5);
+		supports.doesNotIncludeMarginInBodyOffset = (body.offsetTop !== bodyMarginTop);
+
+		return supports;
+
+	})( testElement, div );
+
+	jQuery.extend( support, offsetSupport );
+
 	// Null connected elements to avoid leaks in IE
 	testElement = fragment = select = opt = body = marginDiv = div = input = null;
 

test/unit/offset.js

@@ -267,8 +267,6 @@ testoffset("static", function( jQuery ) {
 testoffset("fixed", function( jQuery ) {
 	expect(30);
 
-	jQuery.offset.initialize();
-
 	var tests = [
 		{ id: "#fixed-1", top: 1001, left: 1001 },
 		{ id: "#fixed-2", top: 1021, left: 1021 }
@@ -403,8 +401,8 @@ testoffset("scroll", function( jQuery, win ) {
 testoffset("body", function( jQuery ) {
 	expect(2);
 
-	equals( jQuery("body").offset().top, 1, "jQuery('#body').offset().top" );
-	equals( jQuery("body").offset().left, 1, "jQuery('#body').offset().left" );
+	equals( jQuery("body").offset().top, 0, "jQuery('#body').offset().top" );
+	equals( jQuery("body").offset().left, 0, "jQuery('#body').offset().left" );
 });
 
 test("Chaining offset(coords) returns jQuery object", function() {