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 {

components/layout_2020/fragment_tree/base_fragment.rs

@@ -102,6 +102,8 @@ bitflags! {
         /// and the fragment can be a flex item. This flag is used to cache items during flex
         /// layout.
         const SIZE_DEPENDS_ON_BLOCK_CONSTRAINTS_AND_CAN_BE_CHILD_OF_FLEX_ITEM = 1 << 8;
+        /// Whether or not the node that created this fragment is the root element.
+        const IS_ROOT_ELEMENT = 1 << 9;
     }
 }
 

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
     }