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/shared/script_layout/wrapper_traits.rs

@@ -390,4 +390,10 @@ pub trait ThreadSafeLayoutElement<'dom>:
     /// of the parent data is fine, since the bottom-up traversal will not process
     /// the parent until all the children have been processed.
     fn is_body_element_of_html_element_root(&self) -> bool;
+
+    /// Returns whether this node is the root element in an HTML document element.
+    ///
+    /// Note that, like `Self::is_body_element_of_html_element_root`, this accesses the parent.
+    /// As in that case, since this is an immutable borrow, we do not violate thread safety.
+    fn is_root(&self) -> bool;
 }