fix: root element not establishing stacking context (#35390) (#36174)

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #35390 
- [X] There are tests for these changes

[Successful WPT
run](https://github.com/reesmichael1/servo/actions/runs/14097679625)
(which includes the new test files)

(I didn't make the formatting changes intentionally--those came from
`mach format` following `mach test-tidy`.)

---------

Signed-off-by: Michael Rees <mrees@noeontheend.com>

components/script/layout_dom/element.rs

@@ -868,6 +868,10 @@ impl<'dom> ThreadSafeLayoutElement<'dom> for ServoThreadSafeLayoutElement<'dom>
     fn is_body_element_of_html_element_root(&self) -> bool {
         self.element.is_html_document_body_element()
     }
+
+    fn is_root(&self) -> bool {
+        self.element.is_root()
+    }
 }
 
 /// This implementation of `::selectors::Element` is used for implementing lazy