Test rejection of selectors with unmatched parentheses

Also reject selectors with unclosed parentheses.

src/RuleSet/DeclarationBlock.php

@@ -88,6 +88,9 @@ public static function parse(ParserState $parserState, ?CSSList $list = null): ?
                         break;
                 }
             } while (!\in_array($nextCharacter, ['{', '}'], true) || \is_string($stringWrapperCharacter));
+            if ($functionNestingLevel !== 0) {
+                throw new UnexpectedTokenException(')', $nextCharacter);
+            }
             $selectors[] = \implode('', $selectorParts); // add final or only selector
             $result->setSelectors($selectors, $list);
             if ($parserState->comes('{')) {

tests/Unit/RuleSet/DeclarationBlockTest.php

@@ -115,6 +115,40 @@ public function parsesTwoCommaSeparatedSelectors(string $firstSelector, string $
         self::assertSame([$firstSelector, $secondSelector], self::getSelectorsAsStrings($subject));
     }
 
+    /**
+     * @return array<non-empty-string, array{0: non-empty-string}>
+     */
+    public static function provideInvalidSelector(): array
+    {
+        // TODO: the `parse` method consumes the first character without inspection,
+        // so the 'lone' test strings are prefixed with a space.
+        return [
+            'lone `(`' => [' ('],
+            'lone `)`' => [' )'],
+            'unclosed `(`' => [':not(#your-mug'],
+            'extra `)`' => [':not(#your-mug))'],
+        ];
+    }
+
+    /**
+     * @test
+     *
+     * @param non-empty-string $selector
+     *
+     * @dataProvider provideInvalidSelector
+     */
+    public function parseSkipsBlockWithInvalidSelector(string $selector): void
+    {
+        static $nextCss = ' .next {}';
+        $css = $selector . ' {}' . $nextCss;
+        $parserState = new ParserState($css, Settings::create());
+
+        $subject = DeclarationBlock::parse($parserState);
+
+        self::assertNull($subject);
+        self::assertTrue($parserState->comes($nextCss));
+    }
+
     /**
      * @return array<string>
      */