Cherry-pick 3651419. rdar://problem/110629287

    [CSSOM] Fix insertion of rule into orphaned style rule
    https://bugs.webkit.org/show_bug.cgi?id=258017
    rdar://110629287

    Reviewed by Antti Koivisto.

    When a StyleRule is orphaned, we don't need any specific mechanism (such as RuleMutationScope)
    and convert it directly to a StyleRuleWithNesting.

    * LayoutTests/fast/css/cssom-mutation-stylerule-expected.html: Added.
    * LayoutTests/fast/css/cssom-mutation-stylerule.html: Added.
    * Source/WebCore/css/CSSStyleRule.cpp:
    (WebCore::CSSStyleRule::insertRule):

    Canonical link: https://commits.webkit.org/265154@main
Identifier: 259548.836@safari-7615-branch

LayoutTests/fast/css/cssom-mutation-stylerule-expected.html

@@ -0,0 +1,2 @@
+
+Test passes if no crash.

LayoutTests/fast/css/cssom-mutation-stylerule.html

@@ -0,0 +1,14 @@
+<style>
+    aaa {
+        color: red;
+    }
+</style>
+<script>
+  onload = () => {
+    let cssStyleRule = document.styleSheets[0].cssRules[0];
+    document.styleSheets[0].deleteRule(0);
+    cssStyleRule.insertRule(`.bar { color: black; }`);
+  };
+</script>
+
+Test passes if no crash.

Source/WebCore/css/CSSStyleRule.cpp

@@ -217,9 +217,14 @@ ExceptionOr<unsigned> CSSStyleRule::insertRule(const String& ruleString, unsigne
         return Exception { HierarchyRequestError };
 
     if (!m_styleRule->isStyleRuleWithNesting()) {
-        // Call the parent rule (or parent stylesheet if top-level) to transform the current StyleRule to StyleRuleWithNesting.
-        auto parent = parentRule();
-        auto styleRuleWithNesting = parent ? parent->prepareChildStyleRuleForNesting(m_styleRule) : styleSheet->prepareChildStyleRuleForNesting(WTFMove(m_styleRule.get()));
+        // Call the parent rule (or parent stylesheet if top-level or nothing if it's an orphaned rule) to transform the current StyleRule to StyleRuleWithNesting.
+        RefPtr<StyleRuleWithNesting> styleRuleWithNesting;
+        if (auto parent = parentRule())
+            styleRuleWithNesting = parent->prepareChildStyleRuleForNesting(m_styleRule);
+        else if (auto parent = parentStyleSheet())
+            styleRuleWithNesting = parent->prepareChildStyleRuleForNesting(WTFMove(m_styleRule.get()));
+        else
+            styleRuleWithNesting = StyleRuleWithNesting::create(WTFMove(m_styleRule.get()));
         ASSERT(styleRuleWithNesting);
         m_styleRule = *styleRuleWithNesting;
     }