Followup to #11469; add test case and clarify code.

src/effects.js

@@ -4,6 +4,7 @@ var elemdisplay = {},
 	iframe, iframeDoc,
 	rfxtypes = /^(?:toggle|show|hide)$/,
 	rfxnum = /^([+\-]=)?([\d+.\-]+)([a-z%]*)$/i,
+	rMarginProp = /^margin/,
 	timerId,
 	fxAttrs = [
 		// height animations
@@ -636,8 +637,8 @@ jQuery.extend( jQuery.fx, {
 
 // Ensure props that can't be negative don't go there on undershoot easing
 jQuery.each( fxAttrs.concat.apply( [], fxAttrs ), function( i, prop ) {
-	// exclude marginTop, marginLeft, marginBottom and marginRight from this list
-	if ( prop.indexOf( "margin" ) ) {
+	// Exclude marginTop, marginLeft, marginBottom and marginRight from this list
+	if ( !rMarginProp.test( prop ) ) {
 		jQuery.fx.step[ prop ] = function( fx ) {
 			jQuery.style( fx.elem, prop, Math.max(0, fx.now) + fx.unit );
 		};

test/unit/effects.js

@@ -263,6 +263,15 @@ test("animate negative height", function() {
 	});
 });
 
+test("animate negative margin", function() {
+	expect(1);
+	stop();
+	jQuery("#foo").animate({ marginTop: -100 }, 100, function() {
+		equal( jQuery(this).css("marginTop"), "-100px", "Verify margin." );
+		start();
+	});
+});
+
 test("animate negative padding", function() {
 	expect(1);
 	stop();