script: Chain up keyboard scrolling to parent <iframe>s (#39469)

When an `<iframe>` cannot scroll because the size of the frame is
greater than or
equal to the size of page contents, chain up the keyboard scroll
operation to the parent frame.

Testing: A new Servo-only WPT tests is added, though needs to be
manually
run with `--product servodriver`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>

components/layout/flow/root.rs

@@ -42,7 +42,7 @@ pub struct BoxTree {
     root: BlockFormattingContext,
 
     /// Whether or not the viewport should be sensitive to scrolling input events in two axes
-    viewport_scroll_sensitivity: AxesScrollSensitivity,
+    pub(crate) viewport_overflow: AxesOverflow,
 }
 
 impl BoxTree {
@@ -65,10 +65,7 @@ impl BoxTree {
             // From https://www.w3.org/TR/css-overflow-3/#overflow-propagation:
             // > If visible is applied to the viewport, it must be interpreted as auto.
             // > If clip is applied to the viewport, it must be interpreted as hidden.
-            viewport_scroll_sensitivity: AxesScrollSensitivity {
-                x: viewport_overflow.x.to_scrollable().into(),
-                y: viewport_overflow.y.to_scrollable().into(),
-            },
+            viewport_overflow: viewport_overflow.to_scrollable(),
         }
     }
 
@@ -266,11 +263,16 @@ impl BoxTree {
             &mut root_fragments,
         );
 
+        let viewport_scroll_sensitivity = AxesScrollSensitivity {
+            x: self.viewport_overflow.x.into(),
+            y: self.viewport_overflow.y.into(),
+        };
+
         FragmentTree::new(
             layout_context,
             root_fragments,
             physical_containing_block,
-            self.viewport_scroll_sensitivity,
+            viewport_scroll_sensitivity,
         )
     }
 }

components/layout/layout_impl.rs

@@ -333,11 +333,17 @@ impl Layout for LayoutThread {
     #[servo_tracing::instrument(skip_all)]
     fn query_scroll_container(
         &self,
-        node: TrustedNodeAddress,
+        node: Option<TrustedNodeAddress>,
         flags: ScrollContainerQueryFlags,
     ) -> Option<ScrollContainerResponse> {
-        let node = unsafe { ServoLayoutNode::new(&node) };
-        process_scroll_container_query(node, flags)
+        let node = unsafe { node.as_ref().map(|node| ServoLayoutNode::new(node)) };
+        let viewport_overflow = self
+            .box_tree
+            .borrow()
+            .as_ref()
+            .expect("Should have a BoxTree for all scroll container queries.")
+            .viewport_overflow;
+        process_scroll_container_query(node, flags, viewport_overflow)
     }
 
     #[servo_tracing::instrument(skip_all)]

components/layout/query.rs

@@ -12,7 +12,7 @@ use euclid::{SideOffsets2D, Size2D};
 use itertools::Itertools;
 use layout_api::wrapper_traits::{LayoutNode, ThreadSafeLayoutElement, ThreadSafeLayoutNode};
 use layout_api::{
-    BoxAreaType, LayoutElementType, LayoutNodeType, OffsetParentResponse,
+    AxesOverflow, BoxAreaType, LayoutElementType, LayoutNodeType, OffsetParentResponse,
     ScrollContainerQueryFlags, ScrollContainerResponse,
 };
 use script::layout_dom::{ServoLayoutNode, ServoThreadSafeLayoutNode};
@@ -676,9 +676,14 @@ pub fn process_offset_parent_query(
 ///
 #[inline]
 pub(crate) fn process_scroll_container_query(
-    node: ServoLayoutNode<'_>,
+    node: Option<ServoLayoutNode<'_>>,
     query_flags: ScrollContainerQueryFlags,
+    viewport_overflow: AxesOverflow,
 ) -> Option<ScrollContainerResponse> {
+    let Some(node) = node else {
+        return Some(ScrollContainerResponse::Viewport(viewport_overflow));
+    };
+
     let layout_data = node.to_threadsafe().inner_layout_data()?;
 
     // 1. If any of the following holds true, return null and terminate this algorithm:
@@ -776,7 +781,7 @@ pub(crate) fn process_scroll_container_query(
 
     match current_position_value {
         Position::Fixed => None,
-        _ => Some(ScrollContainerResponse::Viewport),
+        _ => Some(ScrollContainerResponse::Viewport(viewport_overflow)),
     }
 }