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/style_ext.rs

@@ -746,6 +746,12 @@ impl ComputedValuesExt for ComputedValues {
             return true;
         }
 
+        // From https://www.w3.org/TR/CSS22/visuren.html#z-index:
+        // > The root element forms the root stacking context.
+        if fragment_flags.contains(FragmentFlags::IS_ROOT_ELEMENT) {
+            return true;
+        }
+
         // TODO: We need to handle CSS Contain here.
         false
     }