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>

8 files changed, 53 insertions, 0 deletions

diff --git a/components/layout_2020/dom_traversal.rs b/components/layout_2020/dom_traversal.rs
index 33e500bfe87..bcb6555fd13 100644
--- a/components/layout_2020/dom_traversal.rs
+++ b/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 {
diff --git a/components/layout_2020/fragment_tree/base_fragment.rs b/components/layout_2020/fragment_tree/base_fragment.rs
index 8e529f40ba6..d26ae71264f 100644
--- a/components/layout_2020/fragment_tree/base_fragment.rs
+++ b/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;
     }
 }
 
diff --git a/components/layout_2020/style_ext.rs b/components/layout_2020/style_ext.rs
index 4a8d15b9fd3..f3269fb5461 100644
--- a/components/layout_2020/style_ext.rs
+++ b/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
     }
diff --git a/components/script/layout_dom/element.rs b/components/script/layout_dom/element.rs
index 6a267f16077..85c04039323 100644
--- a/components/script/layout_dom/element.rs
+++ b/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
diff --git a/components/shared/script_layout/wrapper_traits.rs b/components/shared/script_layout/wrapper_traits.rs
index f64ec94a777..3e021052280 100644
--- a/components/shared/script_layout/wrapper_traits.rs
+++ b/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;
 }
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index 5798aa29c28..70903a3077a 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -102180,6 +102180,19 @@
         {}
        ]
       ],
+      "root-element-creates-stacking-context.html": [
+       "be7b027ee791d8d83d6fcecae60654eb22ff960c",
+       [
+        null,
+        [
+         [
+          "/css/CSS2/stacking-context/root-element-creates-stacking-context-ref.html",
+          "=="
+         ]
+        ],
+        {}
+       ]
+      ],
       "zindex-affects-block-in-inline.html": [
        "5a5c9e3dcca6171910f85790e13dc754acda58e0",
        [
@@ -405642,6 +405655,10 @@
        "65138c9a66713e1ed8c2f649751deb1ceb22eac3",
        []
       ],
+      "root-element-creates-stacking-context-ref.html": [
+       "9bb42041498c4a5b2b86e6b282ec49e603a0880b",
+       []
+      ],
       "zindex-affects-block-in-inline-ref.html": [
        "c0fdb338d08198cfd058ccdabd7766a9e57375fd",
        []
diff --git a/tests/wpt/tests/css/CSS2/stacking-context/root-element-creates-stacking-context-ref.html b/tests/wpt/tests/css/CSS2/stacking-context/root-element-creates-stacking-context-ref.html
new file mode 100644
index 00000000000..9bb42041498
--- /dev/null
+++ b/tests/wpt/tests/css/CSS2/stacking-context/root-element-creates-stacking-context-ref.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<body style="width: 100px; margin: 0">
+  <div style="height: 100px; background: green"></div>
+</body>
diff --git a/tests/wpt/tests/css/CSS2/stacking-context/root-element-creates-stacking-context.html b/tests/wpt/tests/css/CSS2/stacking-context/root-element-creates-stacking-context.html
new file mode 100644
index 00000000000..be7b027ee79
--- /dev/null
+++ b/tests/wpt/tests/css/CSS2/stacking-context/root-element-creates-stacking-context.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<html style="width: 0; height: 0; border: 50px solid red">
+  <link rel="help" href="https://drafts.csswg.org/css2/#stacking-context">
+  <link rel="match" href="root-element-creates-stacking-context-ref.html">
+  <link rel="author" title="Michael Rees" href="mailto:mrees@noeontheend.com">
+  <meta name="assert" content="root element forms the root stacking context">
+  <body style="border: 50px solid green; margin: -50px; position: relative; z-index: -1">
+  </body>
+</html>