From 801768386b9fd44f1282834449b4cd1ae1e8d593 Mon Sep 17 00:00:00 2001
From: Jeffery To <jeffery.to@gmail.com>
Date: Mon, 11 Jun 2012 14:17:36 -0400
Subject: [PATCH] Fix #8109, percent animations; fix #11854, wrong this, close
 gh-808

---
 src/effects.js       | 30 ++++++++++++++++++++++++------
 test/unit/effects.js | 18 ++++++++++++++++++
 2 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/src/effects.js b/src/effects.js
index ff1c98d80..7aae8a414 100644
--- a/src/effects.js
+++ b/src/effects.js
@@ -7,20 +7,38 @@ var fxNow, timerId,
 	animationPrefilters = [ defaultPrefilter ],
 	tweeners = {
 		"*": [function( prop, value ) {
-			var end, unit,
+			var end, unit, prevScale,
 				tween = this.createTween( prop, value ),
 				parts = rfxnum.exec( value ),
-				start = tween.cur();
+				start = tween.cur(),
+				scale = 1,
+				target = start;
 
 			if ( parts ) {
 				end = +parts[2];
 				unit = parts[3] || ( jQuery.cssNumber[ prop ] ? "" : "px" );
 
 				// We need to compute starting value
-				if ( unit !== "px" ) {
-					jQuery.style( this, prop, (end || 1) + unit);
-					start = start * (end || 1) / tween.cur() || 0;
-					jQuery.style( this, prop, start + unit);
+				if ( unit !== "px" && start ) {
+					// Iteratively approximate from a nonzero starting point
+					// Prefer the current property, because this process will be trivial if it uses the same units
+					// Fallback to end or a simple constant
+					start = parseFloat( jQuery.style( tween.elem, prop ) ) || end || 1;
+
+					do {
+						// If previous iteration zeroed out, double until we get *something*
+						// Use a string for doubling factor so we don't accidentally see scale as unchanged below
+						prevScale = scale = scale || ".5";
+
+						// Adjust and apply
+						start = start / scale;
+						jQuery.style( tween.elem, prop, start + unit );
+
+						// Update scale, tolerating zeroes from tween.cur()
+						scale = tween.cur() / target;
+
+					// Stop looping if scale is unchanged or we've hit the mark
+					} while ( scale !== 1 && scale !== prevScale );
 				}
 
 				tween.unit = unit;
diff --git a/test/unit/effects.js b/test/unit/effects.js
index 9e244490d..48793e234 100644
--- a/test/unit/effects.js
+++ b/test/unit/effects.js
@@ -1628,4 +1628,22 @@ asyncTest( "multiple unqueued and promise", 4, function() {
 	});
 });
 
+asyncTest( "animate does not change start value for non-px animation (#7109)", 1, function() {
+	var parent = jQuery( "<div><div></div></div>" ).css({ width: 284, height: 1 }).appendTo( "#qunit-fixture" ),
+		child = parent.children().css({ fontSize: "98.6in", width: "0.01em", height: 1 }),
+		actual = parseFloat( child.css( "width" ) ),
+		computed = [];
+
+	child.animate({ width: "0%" }, {
+		duration: 1,
+		step: function() {
+			computed.push( parseFloat( child.css( "width" ) ) );
+		}
+	}).queue( function( next ) {
+		equal( computed[0], actual, "Starting width was unchanged" );
+		next();
+		start();
+	});
+});
+
 } // if ( jQuery.fx )
-- 
2.39.5