CSS: Correct misrepresentation of "auto" horizontal margins as 0

Fixes gh-2237
Closes gh-2276

(cherry picked from commit 214e1634ab9b1d13d53647dd5de3bdf7a091d49c)

Conflicts:
	src/css.js
	src/css/support.js
	test/unit/support.js

2 files changed, 31 insertions, 6 deletions

diff --git a/src/css.js b/src/css.js
index 4fbc6745e..0e5a55610 100644
--- a/src/css.js
+++ b/src/css.js
@@ -350,6 +350,19 @@ jQuery.each( [ "height", "width" ], function( i, name ) {
 	};
 } );
 
+jQuery.cssHooks.marginLeft = addGetHookIf( support.reliableMarginLeft,
+	function( elem, computed ) {
+		if ( computed ) {
+			return ( parseFloat( curCSS( elem, "marginLeft" ) ) ||
+				elem.getBoundingClientRect().left -
+					swap( elem, { marginLeft: 0 }, function() {
+						return elem.getBoundingClientRect().left;
+					} )
+				) + "px";
+		}
+	}
+);
+
 // These hooks are used by animate to expand properties
 jQuery.each( {
 	margin: "",
diff --git a/src/css/support.js b/src/css/support.js
index 08560b42a..f8e02d048 100644
--- a/src/css/support.js
+++ b/src/css/support.js
@@ -6,7 +6,7 @@ define( [
 ], function( jQuery, document, documentElement, support ) {
 
 ( function() {
-	var pixelPositionVal, boxSizingReliableVal, pixelMarginRightVal,
+	var pixelPositionVal, boxSizingReliableVal, pixelMarginRightVal, reliableMarginLeftVal,
 		container = document.createElement( "div" ),
 		div = document.createElement( "div" );
 
@@ -30,16 +30,20 @@ define( [
 	function computeStyleTests() {
 		div.style.cssText =
 			"box-sizing:border-box;" +
-			"display:block;position:absolute;" +
-			"margin:0;margin-top:1%;margin-right:50%;" +
-			"border:1px;padding:1px;" +
-			"top:1%;width:50%;height:4px";
+			"position:relative;display:block;" +
+			"margin:auto;border:1px;padding:1px;" +
+			"top:1%;width:50%";
 		div.innerHTML = "";
 		documentElement.appendChild( container );
 
 		var divStyle = window.getComputedStyle( div );
 		pixelPositionVal = divStyle.top !== "1%";
-		boxSizingReliableVal = divStyle.height === "4px";
+		reliableMarginLeftVal = divStyle.marginLeft === "2px";
+		boxSizingReliableVal = divStyle.width === "4px";
+
+		// Support: Android 4.0 - 4.3 only
+		// Some styles come back with percentage values, even though they shouldn't
+		div.style.marginRight = "50%";
 		pixelMarginRightVal = divStyle.marginRight === "4px";
 
 		documentElement.removeChild( container );
@@ -69,6 +73,14 @@ define( [
 				computeStyleTests();
 			}
 			return pixelMarginRightVal;
+		},
+		reliableMarginLeft: function() {
+
+			// Support: IE <=8 only, Android 4.0 - 4.3 only, Firefox <=3 - 37
+			if ( boxSizingReliableVal == null ) {
+				computeStyleTests();
+			}
+			return reliableMarginLeftVal;
 		}
 	} );
 } )();