CSS: Return undefined for whitespace-only CSS variable values (#5120)

The spec requires that CSS variable values are trimmed. In browsers that do
this - mainly, Safari, but also Firefox if the value only has leading
whitespace - we currently return undefined; in other browsers, we return
an empty string as the logic to fall back to undefined happens before
trimming.

This commit adds another explicit callback to `undefined` to have it consistent
across browsers.

Also, more explicit comments about behaviors we need to work around in various
browsers have been added.

Closes gh-5120
Ref gh-5106

(cherry picked from commit 7eb0019640)

src/css/curCSS.js

@@ -28,17 +28,37 @@ function curCSS( elem, name, computed ) {
 	//   .css('filter') (IE 9 only, trac-12537)
 	//   .css('--customProperty) (gh-3144)
 	if ( computed ) {
+
+		// Support: IE <=9 - 11+
+		// IE only supports `"float"` in `getPropertyValue`; in computed styles
+		// it's only available as `"cssFloat"`. We no longer modify properties
+		// sent to `.css()` apart from camelCasing, so we need to check both.
+		// Normally, this would create difference in behavior: if
+		// `getPropertyValue` returns an empty string, the value returned
+		// by `.css()` would be `undefined`. This is usually the case for
+		// disconnected elements. However, in IE even disconnected elements
+		// with no styles return `"none"` for `getPropertyValue( "float" )`
 		ret = computed.getPropertyValue( name ) || computed[ name ];
 
-		// trim whitespace for custom property (issue gh-4926)
+		if ( isCustomProp && ret ) {
-		if ( isCustomProp && ret !== undefined ) {
 
-			// rtrim treats U+000D CARRIAGE RETURN and U+000C FORM FEED
+			// Support: Firefox 105+, Chrome <=105+
+			// Spec requires trimming whitespace for custom properties (gh-4926).
+			// Firefox only trims leading whitespace. Chrome just collapses
+			// both leading & trailing whitespace to a single space.
+			//
+			// Fall back to `undefined` if empty string returned.
+			// This collapses a missing definition with property defined
+			// and set to an empty string but there's no standard API
+			// allowing us to differentiate them without a performance penalty
+			// and returning `undefined` aligns with older jQuery.
+			//
+			// rtrimCSS treats U+000D CARRIAGE RETURN and U+000C FORM FEED
 			// as whitespace while CSS does not, but this is not a problem
 			// because CSS preprocessing replaces them with U+000A LINE FEED
 			// (which *is* CSS whitespace)
 			// https://www.w3.org/TR/css-syntax-3/#input-preprocessing
-			ret = ret.replace( rtrimCSS, "$1" );
+			ret = ret.replace( rtrimCSS, "$1" ) || undefined;
 		}
 
 		if ( ret === "" && !isAttached( elem ) ) {

test/unit/css.js

@@ -1760,6 +1760,7 @@ QUnit.test( "Do not throw on frame elements from css method (trac-15098)", funct
 			"        --prop10:\f\r\n\t val10 \f\r\n\t;\n" +
 			"        --prop11:\u000C\u000D\u000A\u0009\u0020val11\u0020\u0009\u000A\u000D\u000C;\n" +
 			"        --prop12:\u000Bval12\u000B;\n" +
+			"        --space: ;\n" +
 			"        --empty:;\n" +
 			"    }\n" +
 			"</style>"
@@ -1769,7 +1770,7 @@ QUnit.test( "Do not throw on frame elements from css method (trac-15098)", funct
 			$elem = jQuery( "<div>" ).addClass( "test__customProperties" )
 				.appendTo( "#qunit-fixture" ),
 			webkitOrBlink = /\bsafari\b/i.test( navigator.userAgent ),
-			expected = 19;
+			expected = 20;
 
 		if ( webkitOrBlink ) {
 			expected -= 2;
@@ -1815,6 +1816,7 @@ QUnit.test( "Do not throw on frame elements from css method (trac-15098)", funct
 		assert.equal( $elem.css( "--prop10" ), "val10", "Multiple preceding and following escaped unicode whitespace trimmed" );
 		assert.equal( $elem.css( "--prop11" ), "val11", "Multiple preceding and following unicode whitespace trimmed" );
 		assert.equal( $elem.css( "--prop12" ), "\u000Bval12\u000B", "Multiple preceding and following non-CSS whitespace reserved" );
+		assert.equal( $elem.css( "--space" ), undefined );
 		assert.equal( $elem.css( "--empty" ), undefined );
 		assert.equal( $elem.css( "--nonexistent" ), undefined );
 	} );