From 00231d5d94d3374d53bfe79e04ba253250c73087 Mon Sep 17 00:00:00 2001
From: Richard Gibson <richard.gibson@gmail.com>
Date: Sun, 26 May 2013 16:02:32 -0400
Subject: [PATCH] Fix #13939: Same-unit relative animations

---
 src/effects.js       |  2 +-
 test/unit/effects.js | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/src/effects.js b/src/effects.js
index 982365128..b03036ce3 100644
--- a/src/effects.js
+++ b/src/effects.js
@@ -42,8 +42,8 @@ var fxNow, timerId,
 
 			// Update tween properties
 			if ( parts ) {
+				start = tween.start = +start || +target || 0;
 				tween.unit = unit;
-				tween.start = +start || +target || 0;
 				// If a +=/-= token was provided, we're doing a relative animation
 				tween.end = parts[ 1 ] ?
 					start + ( parts[ 1 ] + 1 ) * parts[ 2 ] :
diff --git a/test/unit/effects.js b/test/unit/effects.js
index a23ff2cca..fc40e49d2 100644
--- a/test/unit/effects.js
+++ b/test/unit/effects.js
@@ -208,6 +208,46 @@ test("animate(Hash, Object, Function)", function() {
 	});
 });
 
+test("animate relative values", function() {
+	stop();
+
+	var value = 40,
+		bases = [ "%", "px", "em" ],
+		adjustments = [ "px", "em" ],
+		container = jQuery("<div></div>")
+			.css({ position: "absolute", height: "50em", width: "50em" }),
+		animations = bases.length * adjustments.length;
+
+	expect( 2 * animations );
+
+	jQuery.each( bases, function( _, baseUnit ) {
+		jQuery.each( adjustments, function( _, adjustUnit ) {
+			var base = value + baseUnit,
+				adjust = { height: "+=2" + adjustUnit, width: "-=2" + adjustUnit },
+				elem = jQuery("<div></div>")
+					.appendTo( container.clone().appendTo("#qunit-fixture") )
+					.css({
+						position: "absolute",
+						height: base,
+						width: value + adjustUnit
+					}),
+				baseScale = elem[ 0 ].offsetHeight / value,
+				adjustScale = elem[ 0 ].offsetWidth / value;
+
+			elem.css( "width", base ).animate( adjust, 100, function() {
+				equal( this.offsetHeight, value * baseScale + 2 * adjustScale,
+					baseUnit + "+=" + adjustUnit );
+				equal( this.offsetWidth, value * baseScale - 2 * adjustScale,
+					baseUnit + "-=" + adjustUnit );
+
+				if ( --animations === 0 ) {
+					start();
+				}
+			});
+		});
+	});
+});
+
 test("animate negative height", function() {
 	expect(1);
 	stop();
-- 
2.39.5