Fix #11311. Percents not pixels for top/left/bottom/right. Closes gh-793

.

src/css.js

@@ -16,6 +16,7 @@ var ralpha = /alpha\([^)]*\)/i,
 
 	cssExpand = jQuery.cssExpand,
 	cssPrefixes = [ "Webkit", "O", "Moz", "ms" ],
+	rposition = /^(top|right|bottom|left)$/,
 
 	curCSS;
 
@@ -242,7 +243,9 @@ if ( document.defaultView && document.defaultView.getComputedStyle ) {
 
 		// If we're not dealing with a regular pixel number
 		// but a number that has a weird ending, we need to convert it to pixels
-		if ( rnumnonpx.test( ret ) ) {
+		// but not position css attributes, as those are proportional to the parent element instead
+		// and we can't measure the parent instead because it might trigger a "stacking dolls" problem
+		if ( rnumnonpx.test( ret ) && !rposition.test( name ) ) {
 
 			// Remember the original values
 			left = style.left;

test/unit/css.js

@@ -538,6 +538,12 @@ test("can't get background-position in IE<9, see #10796", function() {
 	}
 });
 
+test("percentage position properties in IE<9 should not be incorrectly transformed to pixels, see #11311", function() {
+	expect( 1 );
+	var div = jQuery("<div style='position: absolute; width: 1px; height: 20px; bottom:50%;'></div>").appendTo( "#qunit-fixture" );
+	ok( window.getComputedStyle || div.css( "bottom" ) === "50%", "position properties get incorrectly transformed in IE<8, see #11311" );
+});
+
 test("Do not append px to 'fill-opacity' #9548", 1, function() {
 
 	var $div = jQuery("<div>").appendTo("#qunit-fixture");