Fix #11971: force numeric animation start to be numeric, closes gh-836.

src/effects.js

 var fxNow, timerId,
 	rfxtypes = /^(?:toggle|show|hide)$/,
-	rfxnum = /^([+\-]=)?([\d+.\-]+)([a-z%]*)$/i,
+	rfxnum = /^(?:([\-+])=)?([\d+.\-]+)([a-z%]*)$/i,
 	rrun = /queueHooks$/,
 	animationPrefilters = [ defaultPrefilter ],
 	tweeners = {
@@ -8,9 +8,9 @@ var fxNow, timerId,
 			var end, unit, prevScale,
 				tween = this.createTween( prop, value ),
 				parts = rfxnum.exec( value ),
-				start = tween.cur(),
-				scale = 1,
-				target = start;
+				target = tween.cur(),
+				start = +target || 0,
+				scale = 1;
 
 			if ( parts ) {
 				end = +parts[2];
@@ -21,7 +21,7 @@ var fxNow, timerId,
 					// 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.css( tween.elem, prop ) ) || end || 1;
+					start = jQuery.css( tween.elem, prop, true ) || end || 1;
 
 					do {
 						// If previous iteration zeroed out, double until we get *something*
@@ -35,14 +35,14 @@ var fxNow, timerId,
 						// Update scale, tolerating zeroes from tween.cur()
 						scale = tween.cur() / target;
 
-					// Stop looping if scale is unchanged or we've hit the mark
+					// Stop looping if we've hit the mark or scale is unchanged
 					} while ( scale !== 1 && scale !== prevScale );
 				}
 
 				tween.unit = unit;
 				tween.start = start;
 				// If a +=/-= token was provided, we're doing a relative animation
-				tween.end = parts[1] ? start + end * ( parts[1] === "-=" ? -1 : 1 ) : end;
+				tween.end = parts[1] ? start + ( parts[1] + 1 ) * end : end;
 			}
 			return tween;
 		}]

test/data/testsuite.css

@@ -126,3 +126,6 @@ body, div { background: url(http://static.jquery.com/files/rocker/images/logo_jq
 
 /* #10501 */
 section { background:#f0f; display:block; }
+
+/* #11971 */
+#foo { background: url(data/1x1.jpg) right bottom no-repeat; }

test/unit/effects.js

@@ -1677,13 +1677,40 @@ asyncTest( "animate does not change start value for non-px animation (#7109)", 1
 	});
 });
 
-asyncTest("Animation callbacks (#11797)", 12, function() {
+asyncTest( "non-px animation handles non-numeric start (#11971)", 2, function() {
+	var foo = jQuery("#foo"),
+		initial = foo.css("backgroundPositionX");
+
+	foo.animate({ backgroundPositionX: "42%" }, {
+		duration: 1,
+		progress: function( anim, percent ) {
+			if ( percent ) {
+				return;
+			}
+
+			if ( parseFloat( initial ) ) {
+				equal( jQuery.style( this, "backgroundPositionX" ), initial, "Numeric start preserved" );
+			} else {
+				equal( jQuery.style( this, "backgroundPositionX" ), "0%", "Non-numeric start zeroed" );
+			}
+		},
+		done: function() {
+			equal( jQuery.style( this, "backgroundPositionX" ), "42%", "End reached" );
+			start();
+		}
+	});
+});
+
+asyncTest("Animation callbacks (#11797)", 15, function() {
 	var targets = jQuery("#foo").children(),
 		done = false,
 		expectedProgress = 0;
 
 	targets.eq( 0 ).animate( {}, {
 		duration: 1,
+		start: function() {
+			ok( true, "empty: start" );
+		},
 		progress: function( anim, percent ) {
 			equal( percent, 0, "empty: progress 0" );
 		},
@@ -1699,13 +1726,16 @@ asyncTest("Animation callbacks (#11797)", 12, function() {
 		}
 	});
 
-	ok( done, "animation done" );
+	ok( done, "empty: done immediately" );
 
 	done = false;
 	targets.eq( 1 ).animate({
 		opacity: 0
 	}, {
 		duration: 1,
+		start: function() {
+			ok( true, "stopped: start" );
+		},
 		progress: function( anim, percent ) {
 			equal( percent, 0, "stopped: progress 0" );
 		},
@@ -1721,12 +1751,15 @@ asyncTest("Animation callbacks (#11797)", 12, function() {
 		}
 	}).stop();
 
-	ok( done, "animation stopped" );
+	ok( done, "stopped: stopped immediately" );
 
 	targets.eq( 2 ).animate({
 		opacity: 0
 	}, {
 		duration: 1,
+		start: function() {
+			ok( true, "async: start" );
+		},
 		progress: function( anim, percent ) {
 			equal( percent, expectedProgress, "async: progress " + expectedProgress );
 			// once at 0, once at 1