Implement document.scrollingElement (#35994)

<!-- Please describe your changes on the following line: -->
This implements `document.scrollingElement`
(https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement).

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #35700
- [x] There are tests for these changes

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---------

Signed-off-by: JimmyDdotEXE <50691404+JimmyDdotEXE@users.noreply.github.com>

components/script/dom/document.rs

@@ -6369,6 +6369,30 @@ impl DocumentMethods<crate::DomTypeHolder> for Document {
         )
     }
 
+    /// <https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement>
+    fn GetScrollingElement(&self, can_gc: CanGc) -> Option<DomRoot<Element>> {
+        // Step 1. If the Document is in quirks mode, follow these steps:
+        if self.quirks_mode() == QuirksMode::Quirks {
+            // Step 1.1. If the body element exists,
+            if let Some(ref body) = self.GetBody() {
+                let e = body.upcast::<Element>();
+                // and it is not potentially scrollable, return the body element and abort these steps.
+                // For this purpose, a value of overflow:clip on the the body element’s parent element
+                // must be treated as overflow:hidden.
+                if !e.is_potentially_scrollable_body_for_scrolling_element(can_gc) {
+                    return Some(DomRoot::from_ref(e));
+                }
+            }
+
+            // Step 1.2. Return null and abort these steps.
+            return None;
+        }
+
+        // Step 2. If there is a root element, return the root element and abort these steps.
+        // Step 3. Return null.
+        self.GetDocumentElement()
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-document-open
     fn Open(
         &self,

components/script/dom/element.rs

@@ -50,6 +50,7 @@ use style::selector_parser::{
 use style::shared_lock::{Locked, SharedRwLock};
 use style::stylesheets::layer_rule::LayerOrder;
 use style::stylesheets::{CssRuleType, UrlExtraData};
+use style::values::computed::Overflow;
 use style::values::generics::NonNegative;
 use style::values::generics::position::PreferredRatio;
 use style::values::generics::ratio::Ratio;
@@ -455,8 +456,25 @@ impl Element {
             .is_some_and(|s| !s.get_box().clone_display().is_none())
     }
 
-    // https://drafts.csswg.org/cssom-view/#potentially-scrollable
-    fn is_potentially_scrollable_body(&self, can_gc: CanGc) -> bool {
+    /// <https://drafts.csswg.org/cssom-view/#potentially-scrollable>
+    pub(crate) fn is_potentially_scrollable_body(&self, can_gc: CanGc) -> bool {
+        self.is_potentially_scrollable_body_shared_logic(false, can_gc)
+    }
+
+    /// <https://drafts.csswg.org/cssom-view/#potentially-scrollable>
+    pub(crate) fn is_potentially_scrollable_body_for_scrolling_element(
+        &self,
+        can_gc: CanGc,
+    ) -> bool {
+        self.is_potentially_scrollable_body_shared_logic(true, can_gc)
+    }
+
+    /// <https://drafts.csswg.org/cssom-view/#potentially-scrollable>
+    fn is_potentially_scrollable_body_shared_logic(
+        &self,
+        treat_overflow_clip_on_parent_as_hidden: bool,
+        can_gc: CanGc,
+    ) -> bool {
         let node = self.upcast::<Node>();
         debug_assert!(
             node.owner_doc().GetBody().as_deref() == self.downcast::<HTMLElement>(),
@@ -474,9 +492,21 @@ impl Element {
         //     overflow-y properties is neither visible nor clip."
         if let Some(parent) = node.GetParentElement() {
             if let Some(style) = parent.style(can_gc) {
-                if !style.get_box().clone_overflow_x().is_scrollable() &&
-                    !style.get_box().clone_overflow_y().is_scrollable()
-                {
+                let mut overflow_x = style.get_box().clone_overflow_x();
+                let mut overflow_y = style.get_box().clone_overflow_y();
+
+                // This fulfills the 'treat parent element overflow:clip as overflow:hidden' stipulation
+                // from the document.scrollingElement specification.
+                if treat_overflow_clip_on_parent_as_hidden {
+                    if overflow_x == Overflow::Clip {
+                        overflow_x = Overflow::Hidden;
+                    }
+                    if overflow_y == Overflow::Clip {
+                        overflow_y = Overflow::Hidden;
+                    }
+                }
+
+                if !overflow_x.is_scrollable() && !overflow_y.is_scrollable() {
                     return false;
                 }
             };

components/script_bindings/codegen/Bindings.conf

@@ -162,7 +162,7 @@ DOMInterfaces = {
 
 'Document': {
     'additionalTraits': ["crate::interfaces::DocumentHelpers"],
-    'canGc': ['Close', 'CreateElement', 'CreateElementNS', 'ImportNode', 'SetTitle', 'Write', 'Writeln', 'CreateEvent', 'CreateRange', 'Open', 'Open_', 'CreateComment', 'CreateAttribute', 'CreateAttributeNS', 'CreateDocumentFragment', 'CreateTextNode', 'CreateCDATASection', 'CreateProcessingInstruction', 'Prepend', 'Append', 'ReplaceChildren', 'SetBgColor', 'SetFgColor', 'Fonts', 'ElementFromPoint', 'ElementsFromPoint', 'ExitFullscreen', 'CreateExpression', 'CreateNSResolver', 'Evaluate', 'StyleSheets', 'Implementation', 'GetElementsByTagName', 'GetElementsByTagNameNS', 'GetElementsByClassName', 'AdoptNode', 'CreateNodeIterator', 'SetBody', 'GetElementsByName', 'Images', 'Embeds', 'Plugins', 'Links', 'Forms', 'Scripts', 'Anchors', 'Applets', 'Children', 'GetSelection'],
+    'canGc': ['Close', 'CreateElement', 'CreateElementNS', 'ImportNode', 'SetTitle', 'Write', 'Writeln', 'CreateEvent', 'CreateRange', 'Open', 'Open_', 'CreateComment', 'CreateAttribute', 'CreateAttributeNS', 'CreateDocumentFragment', 'CreateTextNode', 'CreateCDATASection', 'CreateProcessingInstruction', 'Prepend', 'Append', 'ReplaceChildren', 'SetBgColor', 'SetFgColor', 'Fonts', 'ElementFromPoint', 'ElementsFromPoint', 'GetScrollingElement', 'ExitFullscreen', 'CreateExpression', 'CreateNSResolver', 'Evaluate', 'StyleSheets', 'Implementation', 'GetElementsByTagName', 'GetElementsByTagNameNS', 'GetElementsByClassName', 'AdoptNode', 'CreateNodeIterator', 'SetBody', 'GetElementsByName', 'Images', 'Embeds', 'Plugins', 'Links', 'Forms', 'Scripts', 'Anchors', 'Applets', 'Children', 'GetSelection'],
 },
 
 'DissimilarOriginWindow': {

components/script_bindings/webidls/Document.webidl

@@ -204,6 +204,16 @@ partial interface Document {
 
 Document includes DocumentOrShadowRoot;
 
+// https://drafts.csswg.org/cssom-view/#extensions-to-the-document-interface
+partial interface Document {
+  // CaretPosition? caretPositionFromPoint(double x, double y, optional CaretPositionFromPointOptions options = {});
+  readonly attribute Element? scrollingElement;
+};
+
+// dictionary CaretPositionFromPointOptions {
+//   sequence<ShadowRoot> shadowRoots = [];
+// };
+
 // https://w3c.github.io/selection-api/#dom-document
 partial interface Document {
   Selection? getSelection();