script: Expose a LayoutNode::is_root_of_user_agent_widget method (#44197)

The main goal here is to have layout not depend on `ServoLayoutElement`
directly and instead use the layout DOM interface exposed by
`layout-api`. 

This change allows layout to determine if replaced content
is the root of a user agent widget without having to access
`ServoDangerousStyleShadowRoot` which isn't really safe to use in layout
code. `LayoutElement::shadow_root()` is now no longer exposed to layout.

Testing: This should not change behavior, so should be covered by
existing test.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/formatting_contexts.rs

@@ -150,34 +150,33 @@ impl IndependentFormattingContext {
         let non_replaced_contents = match contents {
             Contents::Replaced(contents) => {
                 base_fragment_info.flags.insert(FragmentFlags::IS_REPLACED);
+
                 // Some replaced elements can have inner widgets, e.g. `<video controls>`.
-                let widget = Some(node_and_style_info.node)
-                    .filter(|node| node.pseudo_element_chain().is_empty())
-                    .and_then(|node| node.as_element())
-                    .and_then(|element| element.shadow_root())
-                    .is_some_and(|shadow_root| shadow_root.is_ua_widget())
-                    .then(|| {
-                        let widget_info = node_and_style_info
-                            .with_pseudo_element(context, PseudoElement::ServoAnonymousBox)
-                            .expect("Should always be able to construct info for anonymous boxes.");
-                        // Use a block formatting context for the widget, since the display inside is always flow.
-                        let widget_contents = IndependentFormattingContextContents::Flow(
-                            BlockFormattingContext::construct(
-                                context,
-                                &widget_info,
-                                NonReplacedContents::OfElement,
-                                propagated_data,
-                                false, /* is_list_item */
-                            ),
-                        );
-                        let widget_base =
-                            LayoutBoxBase::new((&widget_info).into(), widget_info.style);
-                        ArcRefCell::new(IndependentFormattingContext::new(
-                            widget_base,
-                            widget_contents,
+                let node = node_and_style_info.node;
+                let widget = (node.pseudo_element_chain().is_empty() &&
+                    node.is_root_of_user_agent_widget())
+                .then(|| {
+                    let widget_info = node_and_style_info
+                        .with_pseudo_element(context, PseudoElement::ServoAnonymousBox)
+                        .expect("Should always be able to construct info for anonymous boxes.");
+                    // Use a block formatting context for the widget, since the display inside is always flow.
+                    let widget_contents = IndependentFormattingContextContents::Flow(
+                        BlockFormattingContext::construct(
+                            context,
+                            &widget_info,
+                            NonReplacedContents::OfElement,
                             propagated_data,
-                        ))
-                    });
+                            false, /* is_list_item */
+                        ),
+                    );
+                    let widget_base = LayoutBoxBase::new((&widget_info).into(), widget_info.style);
+                    ArcRefCell::new(IndependentFormattingContext::new(
+                        widget_base,
+                        widget_contents,
+                        propagated_data,
+                    ))
+                });
+
                 return IndependentFormattingContextContents::Replaced(contents, widget);
             },
             Contents::Widget(non_replaced_contents) => {

components/script/dom/node/node.rs

@@ -2368,6 +2368,14 @@ impl<'dom> LayoutDom<'dom, Node> {
     pub(crate) fn is_in_ua_widget(&self) -> bool {
         self.unsafe_get().is_in_ua_widget()
     }
+
+    pub(crate) fn is_root_of_user_agent_widget(&self) -> bool {
+        self.downcast::<Element>().is_some_and(|element| {
+            element
+                .get_shadow_root_for_layout()
+                .is_some_and(|shadow_root| shadow_root.is_user_agent_widget())
+        })
+    }
 }
 
 //

components/script/dom/shadowroot.rs

@@ -654,7 +654,7 @@ impl<'dom> LayoutDom<'dom, ShadowRoot> {
     }
 
     #[inline]
-    pub(crate) fn is_ua_widget(&self) -> bool {
+    pub(crate) fn is_user_agent_widget(&self) -> bool {
         self.unsafe_get().is_user_agent_widget()
     }
 

components/script/layout_dom/servo_dangerous_style_shadow_root.rs

@@ -54,11 +54,3 @@ impl<'dom> TShadowRoot for ServoDangerousStyleShadowRoot<'dom> {
         Some(self.shadow_root.get_style_data_for_layout())
     }
 }
-
-impl<'dom> ServoDangerousStyleShadowRoot<'dom> {
-    /// Whether or not this [`ServoDangerousStyleShadowRoot`] is the root
-    /// of a user agent widget.
-    pub fn is_ua_widget(&self) -> bool {
-        self.shadow_root.is_ua_widget()
-    }
-}

components/script/layout_dom/servo_layout_element.rs

@@ -54,8 +54,10 @@ impl<'dom> ServoLayoutElement<'dom> {
         self.element.is_html_element()
     }
 
-    /// The shadow root this element is a host of.
-    pub fn shadow_root(&self) -> Option<ServoDangerousStyleShadowRoot<'dom>> {
+    /// The shadow root that this [`ServoLayoutElement`] is a host of, if it has one.
+    ///
+    /// Note: This should *not* be exposed to layout as it allows access to an ancestor element.
+    pub(super) fn shadow_root(&self) -> Option<ServoDangerousStyleShadowRoot<'dom>> {
         self.element.get_shadow_root_for_layout().map(Into::into)
     }
 }

components/script/layout_dom/servo_layout_node.rs

@@ -318,6 +318,10 @@ impl<'dom> LayoutNode<'dom> for ServoLayoutNode<'dom> {
     fn is_single_line_text_input(&self) -> bool {
         self.pseudo_element_chain.is_empty() && self.node.is_text_container_of_single_line_input()
     }
+
+    fn is_root_of_user_agent_widget(&self) -> bool {
+        self.node.is_root_of_user_agent_widget()
+    }
 }
 
 impl NodeInfo for ServoLayoutNode<'_> {

components/shared/layout/layout_node.rs

@@ -221,6 +221,9 @@ pub trait LayoutNode<'dom>: Copy + Debug + NodeInfo + Send + Sync {
     /// <https://html.spec.whatwg.org/multipage/#the-input-element-as-a-text-entry-widget>
     fn is_single_line_text_input(&self) -> bool;
 
+    /// Whether or not this [`LayoutNode`] is in a user agent widget shadow DOM.
+    fn is_root_of_user_agent_widget(&self) -> bool;
+
     /// Set whether or not this node has an active pseudo-element style with a `content`
     /// attribute that uses `attr`.
     fn set_uses_content_attribute_with_attr(&self, _uses_content_attribute_with_attr: bool);