1 files changed, 45 insertions, 1 deletions
diff --git a/src/support.js b/src/support.js
index ad1bd9a57..fb730dca7 100644
--- a/src/support.js
+++ b/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;