Make sure that width or height don't animate to a negative value. Fixes #3881.

Author: jeresig

src/fx.js

@@ -432,7 +432,7 @@ jQuery.extend( jQuery.fx, {
 
 		_default: function(fx){
 			if ( fx.elem.style && fx.elem.style[ fx.prop ] != null ) {
-				fx.elem.style[ fx.prop ] = fx.now + fx.unit;
+				fx.elem.style[ fx.prop ] = (fx.prop === "width" || fx.prop === "height" ? Math.max(0, fx.now) : fx.now) + fx.unit;
 			} else {
 				fx.elem[ fx.prop ] = fx.now;
 			}

test/unit/fx.js

@@ -52,6 +52,15 @@ test("animate(Hash, Object, Function)", function() {
 	});
 });
 
+test("animate negative height", function() {
+	expect(1);
+	stop();
+	jQuery("#foo").animate({ height: -100 }, 100, function() {
+		equals( this.offsetHeight, 0, "Verify height." );
+		start();
+	});
+});
+
 /* // This test ends up being flaky depending upon the CPU load
 test("animate option (queue === false)", function () {
 	expect(1);