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/layout_2020/dom_traversal.rs

@@ -105,6 +105,7 @@ where
             if element.is_body_element_of_html_element_root() {
                 flags.insert(FragmentFlags::IS_BODY_ELEMENT_OF_HTML_ELEMENT_ROOT);
             }
+
             match element.get_local_name() {
                 &local_name!("br") => {
                     flags.insert(FragmentFlags::IS_BR_ELEMENT);
@@ -123,6 +124,10 @@ where
             ) {
                 flags.insert(FragmentFlags::IS_TEXT_CONTROL);
             }
+
+            if ThreadSafeLayoutElement::is_root(&element) {
+                flags.insert(FragmentFlags::IS_ROOT_ELEMENT);
+            }
         };
 
         Self {