CSS: Don't read styles.position in the width/height cssHook unless necessary

Current width/height cssHook reads the computed position style even if not
necessary as the browser passes the scrollboxSize support test. That has been
changed.

This commit also makes the scrollboxSize support test in line with all others
(i.e. only return true or false) and changes the variable name in the hook
to make the code clearer.

Fixes gh-4185
Closes gh-4187

src/css.js

@@ -358,10 +358,14 @@ jQuery.each( [ "height", "width" ], function( i, dimension ) {
 		set: function( elem, value, extra ) {
 			var matches,
 				styles = getStyles( elem ),
-				scrollBoxSize = support.scrollboxSize() === styles.position,
+
+				// Only read styles.position if the test has a chance to fail
+				// to avoid forcing a reflow.
+				scrollboxSizeBuggy = !support.scrollboxSize() &&
+					styles.position === "absolute",
 
 				// To avoid forcing a reflow, only fetch boxSizing if we need it (gh-3991)
-				boxSizingNeeded = scrollBoxSize || extra,
+				boxSizingNeeded = scrollboxSizeBuggy || extra,
 				isBorderBox = boxSizingNeeded &&
 					jQuery.css( elem, "boxSizing", false, styles ) === "border-box",
 				subtract = extra ?
@@ -376,7 +380,7 @@ jQuery.each( [ "height", "width" ], function( i, dimension ) {
 
 			// Account for unreliable border-box dimensions by comparing offset* to computed and
 			// faking a content-box to get border and padding (gh-3699)
-			if ( isBorderBox && scrollBoxSize ) {
+			if ( isBorderBox && scrollboxSizeBuggy ) {
 				subtract -= Math.ceil(
 					elem[ "offset" + dimension[ 0 ].toUpperCase() + dimension.slice( 1 ) ] -
 					parseFloat( styles[ dimension ] ) -

src/css/support.js

@@ -46,7 +46,7 @@ define( [
 		// Support: Chrome <=64
 		// Don't get tricked when zoom affects offsetWidth (gh-4029)
 		div.style.position = "absolute";
-		scrollboxSizeVal = roundPixelMeasures( div.offsetWidth / 3 ) === 12 || "absolute";
+		scrollboxSizeVal = roundPixelMeasures( div.offsetWidth / 3 ) === 12;
 
 		documentElement.removeChild( container );
 

test/unit/support.js

@@ -107,7 +107,7 @@ testIframe(
 				"pixelPosition": true,
 				"radioValue": false,
 				"reliableMarginLeft": true,
-				"scrollboxSize": "absolute"
+				"scrollboxSize": false
 			},
 			chrome: {
 				"ajax": true,