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,