Fix algorithm to find balanced var() pairs

lib/resolve-value.js

@@ -14,6 +14,45 @@ function toString(value) {
 	return String(value);
 }
 
+// Check for balanced `var(` and `)` pairs inside `value`, and return the 3 fragments:
+// `body` (inside), `pre` (before), `post` (after) of the found wrapper
+function balancedVar(value) {
+	var match = balanced('(', ')', value)
+	if (match) {
+		// Check if it was prepended with var
+		if (/(?:^|[^\w-])var$/.test(match.pre)) {
+			// Remove the var from the end of pre
+			return {
+				pre: match.pre.slice(0, -3),
+				body: match.body,
+				post: match.post
+			}
+		} else {
+			// Check inside body
+			var bodyMatch = balancedVar(match.body)
+			if (bodyMatch) {
+				// Reconstruct pre and post
+				return {
+					pre: match.pre + '(' + bodyMatch.pre,
+					body: bodyMatch.body,
+					post: bodyMatch.post + ')' + match.post
+				}
+			} else {
+				// Check inside post
+				var postMatch = balancedVar(match.post)
+				if (postMatch) {
+					// Reconstruct pre
+					return {
+						pre: match.pre + '(' + match.body + ')' + postMatch.pre,
+						body: postMatch.body,
+						post: postMatch.post
+					}
+				}
+			}
+		}
+	}
+}
+
 // Pass in a value string to parse/resolve and a map of available values
 // and we can figure out the final value
 //
@@ -34,7 +73,7 @@ var resolveValue = function(decl, map, /*optional*/ignorePseudoScope, /*internal
 	// Create a temporary variable, storing resultantValue variable value
 	var remainingVariableValue = resultantValue;
 	// Use balanced lib to find var() declarations and store variable names
-	while ((matchingVarDecl = balanced('var(', ')', remainingVariableValue))) {
+	while ((matchingVarDecl = balancedVar(remainingVariableValue))) {
 		// Split at the comma to find variable name and fallback value
 		// There may be other commas in the values so this isn't necessarily just 2 pieces
 		var variableFallbackSplitPieces = matchingVarDecl.body.split(',');
@@ -61,7 +100,7 @@ var resolveValue = function(decl, map, /*optional*/ignorePseudoScope, /*internal
 	// var() = var( <custom-property-name> [, <any-value> ]? )
 	// matches `name[, fallback]`, captures "name" and "fallback"
 	// See: http://dev.w3.org/csswg/css-variables/#funcdef-var
-	while ((matchingVarDecl = balanced('var(', ')', resultantValue))) {
+	while ((matchingVarDecl = balancedVar(resultantValue))) {
 		var matchingVarDeclMapItem = undefined;
 
 		// Split at the comma to find variable name and fallback value

test/fixtures/fallback-with-parenthesis.css

@@ -0,0 +1,6 @@
+:root {
+  --box-shadow: 0px 2px 2px 0px #fff;
+}
+.box {
+  box-shadow: var(--box-shadow, 0px 2px 8px 0px rgba(0, 0, 0, 0.5));
+}

test/fixtures/fallback-with-parenthesis.expected.css

@@ -0,0 +1,3 @@
+.box {
+  box-shadow: 0px 2px 2px 0px #fff;
+}

test/test.js

@@ -89,6 +89,7 @@ describe('postcss-css-variables', function() {
 
 
 	test('should work with variables declared in root', 'root-variable');
+	test('should work with variables with parenthesis in fallback', 'fallback-with-parenthesis');
 
 	test('should work with locally scoped variable in a non-root rule', 'local-variable-non-root');