CSS: Avoid forcing a reflow in width/height getters unless necessary

mgol · web-flow · commit a0abd15b9e5a · 2019-03-18T18:44:43.000+01:00
Fixes gh-4322 Closes gh-4325 Ref gh-3991 Ref gh-4010 Ref gh-4185 Ref gh-4187
diff --git a/src/css.js b/src/css.js
@@ -118,9 +118,15 @@ function getWidthOrHeight( elem, dimension, extra ) {
 
 	// Start with computed style
 	var styles = getStyles( elem ),
-		val = curCSS( elem, dimension, styles ),
-		isBorderBox = jQuery.css( elem, "boxSizing", false, styles ) === "border-box",
+
+		// To avoid forcing a reflow, only fetch boxSizing if we need it (gh-4322).
+		// Fake content-box until we know it's needed to know the true value.
+		boxSizingNeeded = !support.boxSizingReliable() || extra,
+		isBorderBox = boxSizingNeeded &&
+			jQuery.css( elem, "boxSizing", false, styles ) === "border-box",
 		valueIsBorderBox = isBorderBox,
+
+		val = curCSS( elem, dimension, styles ),
 		offsetProp = "offset" + dimension[ 0 ].toUpperCase() + dimension.slice( 1 );
 
 	// Support: Firefox <=54
@@ -141,10 +147,13 @@ function getWidthOrHeight( elem, dimension, extra ) {
 	// Also use offsetWidth/offsetHeight for when box sizing is unreliable
 	// We use getClientRects() to check for hidden/disconnected.
 	// In those cases, the computed value can be trusted to be border-box
-	if ( ( isBorderBox && !support.boxSizingReliable() || val === "auto" ||
+	if ( ( !support.boxSizingReliable() && isBorderBox ||
+		val === "auto" ||
 		!parseFloat( val ) && jQuery.css( elem, "display", false, styles ) === "inline" ) &&
 		elem.getClientRects().length ) {
 
+		isBorderBox = jQuery.css( elem, "boxSizing", false, styles ) === "border-box";
+
 		// Where available, offsetWidth/offsetHeight approximate border box dimensions.
 		// Where not available (e.g., SVG), assume unreliable box-sizing and interpret the
 		// retrieved value as a content box dimension.
diff --git a/test/unit/dimensions.js b/test/unit/dimensions.js
@@ -397,17 +397,17 @@ QUnit.test( "SVG dimensions (border-box)", function( assert ) {
 
 	var svg = jQuery( "<svg style='width: 100px; height: 100px; box-sizing: border-box; border: 1px solid white; padding: 2px; margin: 3px'></svg>" ).appendTo( "#qunit-fixture" );
 
-	assert.equal( svg.width(), 94 );
-	assert.equal( svg.height(), 94 );
+	assert.equal( svg.width(), 94, "width" );
+	assert.equal( svg.height(), 94, "height" );
 
-	assert.equal( svg.innerWidth(), 98 );
-	assert.equal( svg.innerHeight(), 98 );
+	assert.equal( svg.innerWidth(), 98, "innerWidth" );
+	assert.equal( svg.innerHeight(), 98, "innerHeight" );
 
-	assert.equal( svg.outerWidth(), 100 );
-	assert.equal( svg.outerHeight(), 100 );
+	assert.equal( svg.outerWidth(), 100, "outerWidth" );
+	assert.equal( svg.outerHeight(), 100, "outerHeight" );
 
-	assert.equal( svg.outerWidth( true ), 106 );
-	assert.equal( svg.outerHeight( true ), 106 );
+	assert.equal( svg.outerWidth( true ), 106, "outerWidth( true )" );
+	assert.equal( svg.outerHeight( true ), 106, "outerHeight( true )" );
 
 	svg.remove();
 } );
