From 354f6036f251a3ce9b24cd7b228b4c7a79001520 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Micha=C5=82=20Go=C5=82=C4=99biowski-Owczarek?=
 <m.goleb@gmail.com>
Date: Mon, 8 Oct 2018 18:25:15 +0200
Subject: [PATCH] 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           | 10 +++++++---
 src/css/support.js   |  2 +-
 test/unit/support.js |  2 +-
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/css.js b/src/css.js
index aea0ef2c5..cc873994b 100644
--- a/src/css.js
+++ b/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 ] ) -
diff --git a/src/css/support.js b/src/css/support.js
index 12706f7e9..9c4da57d9 100644
--- a/src/css/support.js
+++ b/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 );
 
diff --git a/test/unit/support.js b/test/unit/support.js
index 0983d2295..6019819c5 100644
--- a/test/unit/support.js
+++ b/test/unit/support.js
@@ -107,7 +107,7 @@ testIframe(
 				"pixelPosition": true,
 				"radioValue": false,
 				"reliableMarginLeft": true,
-				"scrollboxSize": "absolute"
+				"scrollboxSize": false
 			},
 			chrome: {
 				"ajax": true,
-- 
2.39.5