Fix another deadlock

Author: SimonSapin

components/script/dom/cssrulelist.rs

@@ -90,9 +90,12 @@ impl CSSRuleList {
         let index = idx as usize;
 
         let parent_stylesheet = self.parent_stylesheet.style_stylesheet();
-        let mut guard = parent_stylesheet.shared_lock.write();
-        let new_rule = css_rules.write_with(&mut guard)
-            .insert_rule(rule, parent_stylesheet, index, nested)?;
+        let new_rule = {
+            let mut guard = parent_stylesheet.shared_lock.write();
+            css_rules.write_with(&mut guard).insert_rule(rule, parent_stylesheet, index, nested)?
+            // Drop `guard` here,
+            // CSSRule::new_specific re-acquires the lock for @support and @media.
+        };
 
         let parent_stylesheet = &*self.parent_stylesheet;
         let dom_rule = CSSRule::new_specific(&window, parent_stylesheet, new_rule);