script: Fire scroll event whenever JS scrolled (#38321)

Implement JS scroll event firing compliant to
https://drafts.csswg.org/cssom-view/#scrolling-events. Basically
whenever, the an element or the viewport is scrolled, we will fire a
scroll event. The changes push a scroll event whenever an API causes a
scroll position to change.

Testing: New WPT tests for basic APIs.
Part of: https://github.com/servo/servo/issues/31665

---------

Signed-off-by: Jo Steven Novaryo <jo.steven.novaryo@huawei.com>

components/layout/layout_impl.rs

@@ -651,18 +651,30 @@ impl LayoutThread {
         );
     }
 
+    /// Checks whether we need to update the scroll node, and report whether the
+    /// node is scrolled. We need to update the scroll node whenever it is requested.
+    fn handle_update_scroll_node_request(&self, reflow_request: &ReflowRequest) -> bool {
+        if let ReflowGoal::UpdateScrollNode(external_scroll_id, offset) = reflow_request.reflow_goal
+        {
+            self.set_scroll_offset_from_script(external_scroll_id, offset)
+        } else {
+            false
+        }
+    }
+
     /// The high-level routine that performs layout.
     #[servo_tracing::instrument(skip_all)]
     fn handle_reflow(&mut self, mut reflow_request: ReflowRequest) -> Option<ReflowResult> {
         self.maybe_print_reflow_event(&reflow_request);
 
         if self.can_skip_reflow_request_entirely(&reflow_request) {
-            if let ReflowGoal::UpdateScrollNode(external_scroll_id, offset) =
-                reflow_request.reflow_goal
-            {
-                self.set_scroll_offset_from_script(external_scroll_id, offset);
-            }
-            return None;
+            // We could skip the layout, but we might need to update the scroll node.
+            let update_scroll_reflow_target_scrolled =
+                self.handle_update_scroll_node_request(&reflow_request);
+
+            return Some(ReflowResult::new_without_relayout(
+                update_scroll_reflow_target_scrolled,
+            ));
         }
 
         let document = unsafe { ServoLayoutNode::new(&reflow_request.document) };
@@ -692,10 +704,8 @@ impl LayoutThread {
         self.build_stacking_context_tree(&reflow_request, damage);
         let built_display_list = self.build_display_list(&reflow_request, damage, &image_resolver);
 
-        if let ReflowGoal::UpdateScrollNode(external_scroll_id, offset) = reflow_request.reflow_goal
-        {
-            self.set_scroll_offset_from_script(external_scroll_id, offset);
-        }
+        let update_scroll_reflow_target_scrolled =
+            self.handle_update_scroll_node_request(&reflow_request);
 
         let pending_images = std::mem::take(&mut *image_resolver.pending_images.lock());
         let pending_rasterization_images =
@@ -705,7 +715,9 @@ impl LayoutThread {
             built_display_list,
             pending_images,
             pending_rasterization_images,
-            iframe_sizes,
+            iframe_sizes: Some(iframe_sizes),
+            update_scroll_reflow_target_scrolled,
+            processed_relayout: true,
         })
     }
 
@@ -1093,10 +1105,10 @@ impl LayoutThread {
         &self,
         external_scroll_id: ExternalScrollId,
         offset: LayoutVector2D,
-    ) {
+    ) -> bool {
         let mut stacking_context_tree = self.stacking_context_tree.borrow_mut();
         let Some(stacking_context_tree) = stacking_context_tree.as_mut() else {
-            return;
+            return false;
         };
 
         if let Some(offset) = stacking_context_tree
@@ -1114,6 +1126,9 @@ impl LayoutThread {
                 offset,
                 external_scroll_id,
             );
+            true
+        } else {
+            false
         }
     }
 

components/script/dom/document.rs

@@ -2413,24 +2413,23 @@ impl Document {
         // Step 2
         // > For each item target in doc’s pending scroll event targets, in the order they
         // > were added to the list, run these substeps:
-        for target in self.pending_scroll_event_targets.borrow().iter() {
-            // Step 2.1
-            // > If target is a Document, fire an event named scroll that bubbles at target.
+        // Step 3.
+        // > Empty doc’s pending scroll event targets.
+        // Since the scroll event callback could trigger another scroll event, we are taking all of the
+        // current scroll event to avoid borrow checking error.
+        rooted_vec!(let notify_list <- self.pending_scroll_event_targets.take().into_iter());
+        for target in notify_list.iter() {
             if target.downcast::<Document>().is_some() {
+                // Step 2.1
+                // > If target is a Document, fire an event named scroll that bubbles at target.
                 target.fire_bubbling_event(Atom::from("scroll"), can_gc);
-            }
-
-            // Step 2.2
-            // > Otherwise, fire an event named scroll at target.
-            if target.downcast::<Element>().is_some() {
+            } else if target.downcast::<Element>().is_some() {
+                // Step 2.2
+                // > Otherwise, fire an event named scroll at target.
                 target.fire_event(Atom::from("scroll"), can_gc);
             }
         }
 
-        // Step 3.
-        // > Empty doc’s pending scroll event targets.
-        self.pending_scroll_event_targets.borrow_mut().clear();
-
         // Step 4.
         // > Run the steps to dispatch pending scrollsnapchange events for doc.
         // TODO(#7673): Implement scroll snapping
@@ -3700,7 +3699,9 @@ impl Document {
             receiver.recv().unwrap();
         }
 
-        self.window().reflow(ReflowGoal::UpdateTheRendering, can_gc)
+        self.window()
+            .reflow(ReflowGoal::UpdateTheRendering, can_gc)
+            .reflow_issued
     }
 
     pub(crate) fn id_map(&self) -> Ref<HashMapTracedValues<Atom, Vec<Dom<Element>>>> {

components/script/dom/element.rs

@@ -2476,6 +2476,7 @@ impl Element {
     }
 
     // https://drafts.csswg.org/cssom-view/#dom-element-scroll
+    // TODO(stevennovaryo): Need to update the scroll API to follow the spec since it is quite outdated.
     pub(crate) fn scroll(&self, x_: f64, y_: f64, behavior: ScrollBehavior, can_gc: CanGc) {
         // Step 1.2 or 2.3
         let x = if x_.is_finite() { x_ } else { 0.0f64 };
@@ -2524,7 +2525,7 @@ impl Element {
         }
 
         // Step 11
-        win.scroll_node(node, x, y, behavior, can_gc);
+        win.scroll_an_element(self, x, y, behavior, can_gc);
     }
 
     /// <https://html.spec.whatwg.org/multipage/#fragment-parsing-algorithm-steps>
@@ -3180,6 +3181,7 @@ impl ElementMethods<crate::DomTypeHolder> for Element {
     }
 
     // https://drafts.csswg.org/cssom-view/#dom-element-scrolltop
+    // TODO(stevennovaryo): Need to update the scroll API to follow the spec since it is quite outdated.
     fn SetScrollTop(&self, y_: f64, can_gc: CanGc) {
         let behavior = ScrollBehavior::Auto;
 
@@ -3229,7 +3231,7 @@ impl ElementMethods<crate::DomTypeHolder> for Element {
         }
 
         // Step 11
-        win.scroll_node(node, self.ScrollLeft(can_gc), y, behavior, can_gc);
+        win.scroll_an_element(self, self.ScrollLeft(can_gc), y, behavior, can_gc);
     }
 
     // https://drafts.csswg.org/cssom-view/#dom-element-scrolltop
@@ -3329,7 +3331,7 @@ impl ElementMethods<crate::DomTypeHolder> for Element {
         }
 
         // Step 11
-        win.scroll_node(node, x, self.ScrollTop(can_gc), behavior, can_gc);
+        win.scroll_an_element(self, x, self.ScrollTop(can_gc), behavior, can_gc);
     }
 
     // https://drafts.csswg.org/cssom-view/#dom-element-scrollwidth

components/script/dom/window.rs

@@ -225,6 +225,25 @@ impl LayoutBlocker {
 
 type PendingImageRasterizationKey = (PendingImageId, DeviceIntSize);
 
+/// Feedbacks of the reflow that is required by the one who is initiating the reflow.
+pub(crate) struct WindowReflowResult {
+    /// Whether the reflow actually happened and it sends a new display list to the embedder.
+    pub reflow_issued: bool,
+    /// Whether the reflow is for [ReflowGoal::UpdateScrollNode] and the target is scrolled.
+    /// Specifically, a node is scrolled whenever the scroll position of it changes. Note
+    /// that reflow that is cancalled would not scroll the target.
+    pub update_scroll_reflow_target_scrolled: bool,
+}
+
+impl WindowReflowResult {
+    fn new_empty() -> Self {
+        WindowReflowResult {
+            reflow_issued: false,
+            update_scroll_reflow_target_scrolled: false,
+        }
+    }
+}
+
 #[dom_struct]
 pub(crate) struct Window {
     globalscope: GlobalScope,
@@ -2145,16 +2164,30 @@ impl Window {
         y: f32,
         scroll_id: ExternalScrollId,
         _behavior: ScrollBehavior,
-        _element: Option<&Element>,
+        element: Option<&Element>,
         can_gc: CanGc,
     ) {
         // TODO Step 1
         // TODO(mrobinson, #18709): Add smooth scrolling support to WebRender so that we can
         // properly process ScrollBehavior here.
-        self.reflow(
+        let WindowReflowResult {
+            update_scroll_reflow_target_scrolled,
+            ..
+        } = self.reflow(
             ReflowGoal::UpdateScrollNode(scroll_id, Vector2D::new(x, y)),
             can_gc,
         );
+
+        // > If the scroll position did not change as a result of the user interaction or programmatic
+        // > invocation, where no translations were applied as a result, then no scrollend event fires
+        // > because no scrolling occurred.
+        // Even though the note mention the scrollend, it is relevant to the scroll as well.
+        if update_scroll_reflow_target_scrolled {
+            match element {
+                Some(el) => self.Document().handle_element_scroll_event(el),
+                None => self.Document().handle_viewport_scroll_event(),
+            };
+        }
     }
 
     pub(crate) fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
@@ -2183,11 +2216,9 @@ impl Window {
     /// no reflow is performed. If reflow is suppressed, no reflow will be performed for ForDisplay
     /// goals.
     ///
-    /// Returns true if layout actually happened and it sent a new display list to the renderer.
-    ///
     /// NOTE: This method should almost never be called directly! Layout and rendering updates should
     /// happen as part of the HTML event loop via *update the rendering*.
-    fn force_reflow(&self, reflow_goal: ReflowGoal) -> bool {
+    fn force_reflow(&self, reflow_goal: ReflowGoal) -> WindowReflowResult {
         let document = self.Document();
         document.ensure_safe_to_run_script_or_layout();
 
@@ -2199,7 +2230,7 @@ impl Window {
             self.layout_blocker.get().layout_blocked()
         {
             debug!("Suppressing pre-load-event reflow pipeline {pipeline_id}");
-            return false;
+            return WindowReflowResult::new_empty();
         }
 
         debug!("script: performing reflow for goal {reflow_goal:?}");
@@ -2251,25 +2282,36 @@ impl Window {
         };
 
         let Some(results) = self.layout.borrow_mut().reflow(reflow) else {
-            return false;
+            return WindowReflowResult::new_empty();
         };
 
-        debug!("script: layout complete");
-        if let Some(marker) = marker {
-            self.emit_timeline_marker(marker.end());
+        // We are maintaining the previous behavior of layout where we are skipping these behavior if we are not
+        // doing layout calculation.
+        if results.processed_relayout {
+            debug!("script: layout complete");
+            if let Some(marker) = marker {
+                self.emit_timeline_marker(marker.end());
+            }
+
+            self.handle_pending_images_post_reflow(
+                results.pending_images,
+                results.pending_rasterization_images,
+            );
+            if let Some(iframe_sizes) = results.iframe_sizes {
+                document
+                    .iframes_mut()
+                    .handle_new_iframe_sizes_after_layout(self, iframe_sizes);
+            }
+            document.update_animations_post_reflow();
+            self.update_constellation_epoch();
+        } else {
+            debug!("script: layout-side reflow finished without relayout");
         }
 
-        self.handle_pending_images_post_reflow(
-            results.pending_images,
-            results.pending_rasterization_images,
-        );
-        document
-            .iframes_mut()
-            .handle_new_iframe_sizes_after_layout(self, results.iframe_sizes);
-        document.update_animations_post_reflow();
-        self.update_constellation_epoch();
-
-        results.built_display_list
+        WindowReflowResult {
+            reflow_issued: results.built_display_list,
+            update_scroll_reflow_target_scrolled: results.update_scroll_reflow_target_scrolled,
+        }
     }
 
     /// Reflows the page if it's possible to do so and the page is dirty. Returns true if layout
@@ -2278,10 +2320,10 @@ impl Window {
     /// NOTE: This method should almost never be called directly! Layout and rendering updates
     /// should happen as part of the HTML event loop via *update the rendering*. Currerntly, the
     /// only exceptions are script queries and scroll requests.
-    pub(crate) fn reflow(&self, reflow_goal: ReflowGoal, can_gc: CanGc) -> bool {
+    pub(crate) fn reflow(&self, reflow_goal: ReflowGoal, can_gc: CanGc) -> WindowReflowResult {
         // Never reflow inactive Documents.
         if !self.Document().is_fully_active() {
-            return false;
+            return WindowReflowResult::new_empty();
         }
 
         // Count the pending web fonts before layout, in case a font loads during the layout.
@@ -2290,7 +2332,7 @@ impl Window {
         self.Document().ensure_safe_to_run_script_or_layout();
 
         let updating_the_rendering = reflow_goal == ReflowGoal::UpdateTheRendering;
-        let issued_reflow = self.force_reflow(reflow_goal);
+        let reflow_result = self.force_reflow(reflow_goal);
 
         let document = self.Document();
         let font_face_set = document.Fonts(can_gc);
@@ -2348,7 +2390,7 @@ impl Window {
             }
         }
 
-        issued_reflow
+        reflow_result
     }
 
     /// If parsing has taken a long time and reflows are still waiting for the `load` event,
@@ -2428,8 +2470,9 @@ impl Window {
         let _ = receiver.recv();
     }
 
-    pub(crate) fn layout_reflow(&self, query_msg: QueryMsg, can_gc: CanGc) -> bool {
-        self.reflow(ReflowGoal::LayoutQuery(query_msg), can_gc)
+    /// Trigger a reflow that is required by a certain queries.
+    pub(crate) fn layout_reflow(&self, query_msg: QueryMsg, can_gc: CanGc) {
+        self.reflow(ReflowGoal::LayoutQuery(query_msg), can_gc);
     }
 
     pub(crate) fn resolved_font_style_query(
@@ -2507,33 +2550,46 @@ impl Window {
         can_gc: CanGc,
     ) -> Vector2D<f32, LayoutPixel> {
         self.layout_reflow(QueryMsg::ScrollingAreaOrOffsetQuery, can_gc);
+        self.scroll_offset_query_with_external_scroll_id_no_reflow(external_scroll_id)
+    }
+
+    fn scroll_offset_query_with_external_scroll_id_no_reflow(
+        &self,
+        external_scroll_id: ExternalScrollId,
+    ) -> Vector2D<f32, LayoutPixel> {
         self.layout
             .borrow()
             .scroll_offset(external_scroll_id)
             .unwrap_or_default()
     }
 
-    // https://drafts.csswg.org/cssom-view/#element-scrolling-members
-    pub(crate) fn scroll_node(
+    /// <https://drafts.csswg.org/cssom-view/#scroll-an-element>
+    // TODO(stevennovaryo): Need to update the scroll API to follow the spec since it is quite outdated.
+    pub(crate) fn scroll_an_element(
         &self,
-        node: &Node,
+        element: &Element,
         x_: f64,
         y_: f64,
         behavior: ScrollBehavior,
         can_gc: CanGc,
     ) {
         let scroll_id = ExternalScrollId(
-            combine_id_with_fragment_type(node.to_opaque().id(), FragmentType::FragmentBody),
+            combine_id_with_fragment_type(
+                element.upcast::<Node>().to_opaque().id(),
+                FragmentType::FragmentBody,
+            ),
             self.pipeline_id().into(),
         );
 
-        // Step 12
+        // Step 6.
+        // > Perform a scroll of box to position, element as the associated element and behavior as
+        // > the scroll behavior.
         self.perform_a_scroll(
             x_.to_f32().unwrap_or(0.0f32),
             y_.to_f32().unwrap_or(0.0f32),
             scroll_id,
             behavior,
-            None,
+            Some(element),
             can_gc,
         );
     }

components/script/script_thread.rs

@@ -1295,8 +1295,14 @@ impl ScriptThread {
             self.process_pending_input_events(*pipeline_id, can_gc);
 
             // > 8. For each doc of docs, run the resize steps for doc. [CSSOMVIEW]
-            if document.window().run_the_resize_steps(can_gc) {
-                // Evaluate media queries and report changes.
+            let resized = document.window().run_the_resize_steps(can_gc);
+
+            // > 9. For each doc of docs, run the scroll steps for doc.
+            document.run_the_scroll_steps(can_gc);
+
+            // Media queries is only relevant when there are resizing.
+            if resized {
+                // 10. For each doc of docs, evaluate media queries and report changes for doc.
                 document
                     .window()
                     .evaluate_media_queries_and_report_changes(can_gc);
@@ -1306,9 +1312,6 @@ impl ScriptThread {
                 document.react_to_environment_changes()
             }
 
-            // > 9. For each doc of docs, run the scroll steps for doc.
-            document.run_the_scroll_steps(can_gc);
-
             // > 11. For each doc of docs, update animations and send events for doc, passing
             // > in relative high resolution time given frameTimestamp and doc's relevant
             // > global object as the timestamp [WEBANIMATIONS]

components/shared/layout/lib.rs

@@ -344,7 +344,8 @@ pub enum ReflowGoal {
     LayoutQuery(QueryMsg),
 
     /// Tells layout about a single new scrolling offset from the script. The rest will
-    /// remain untouched and layout won't forward this back to script.
+    /// remain untouched. Layout will forward whether the element is scrolled through
+    /// [ReflowResult].
     UpdateScrollNode(ExternalScrollId, LayoutVector2D),
 }
 
@@ -391,8 +392,28 @@ pub struct ReflowResult {
     pub pending_rasterization_images: Vec<PendingRasterizationImage>,
     /// The list of iframes in this layout and their sizes, used in order
     /// to communicate them with the Constellation and also the `Window`
-    /// element of their content pages.
-    pub iframe_sizes: IFrameSizes,
+    /// element of their content pages. Returning None if incremental reflow
+    /// finished before reaching this stage of the layout. I.e., no update
+    /// required.
+    pub iframe_sizes: Option<IFrameSizes>,
+    /// Whether the reflow is for [ReflowGoal::UpdateScrollNode] and the target is scrolled.
+    /// Specifically, a node is scrolled whenever the scroll position of it changes.
+    pub update_scroll_reflow_target_scrolled: bool,
+    /// Do the reflow results in a new component within layout. Incremental layout could be
+    /// skipped if it is deemed unnecessary or the required component is not ready to be
+    /// processed.
+    pub processed_relayout: bool,
+}
+
+impl ReflowResult {
+    /// In incremental reflow, we could skip the layout calculation completely, if it is deemed
+    /// unecessary. In those cases, many of the [ReflowResult] would be irrelevant.
+    pub fn new_without_relayout(update_scroll_reflow_target_scrolled: bool) -> Self {
+        ReflowResult {
+            update_scroll_reflow_target_scrolled,
+            ..Default::default()
+        }
+    }
 }
 
 /// Information needed for a script-initiated reflow that requires a restyle

tests/wpt/meta/MANIFEST.json

@@ -467655,6 +467655,10 @@
        "5df4fa279363798f2f471467ed674c092c7a1749",
        []
       ],
+      "large-dimension-document.sub.html": [
+       "5d29f7be0bbb77e6303598aef833a810e84b6e80",
+       []
+      ],
       "prefixed-animation-event-tests.js": [
        "021b6bb9dfdc422d1a6c3c9c4a1a039f89a901d5",
        []
@@ -633745,6 +633749,20 @@
         }
        ]
       ],
+      "scroll-event-fired-to-element.html": [
+       "3484a2ec92b2bd8faa1d5147568f13a83b8c59d9",
+       [
+        null,
+        {}
+       ]
+      ],
+      "scroll-event-fired-to-iframe.html": [
+       "2ff3de79d33fa0a676a0eda4f109b6d0e634cc9d",
+       [
+        null,
+        {}
+       ]
+      ],
       "scrollend-event-fired-after-sequence-of-scrolls.tentative.html": [
        "dab6dcc9bd8d67a514ca95daf3e1d64b10a45188",
        [

tests/wpt/meta/html/webappapis/scripting/event-loops/new-scroll-event-dispatched-at-next-updating-rendering-time.html.ini

@@ -1,4 +0,0 @@
-[new-scroll-event-dispatched-at-next-updating-rendering-time.html]
-  expected: TIMEOUT
-  [new-scroll-event-dispatched-at-next-updating-rendering-time]
-    expected: TIMEOUT

tests/wpt/tests/dom/events/resources/large-dimension-document.sub.html

@@ -0,0 +1,5 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<body>
+    <div style="width: 10000px; height: 10000px;"></div>
+</body>

tests/wpt/tests/dom/events/scrolling/scroll-event-fired-to-element.html

@@ -0,0 +1,100 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Scroll event should behave correctly for Element.offsetTop and Element.offsetLeft</title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script src="scroll_support.js"></script>
+<link rel="author" title="Jo Steven Novaryo" href="mailto:jo.steven.novaryo@huawei.com">
+<link rel="help" href="https://drafts.csswg.org/cssom-view/#extensions-to-the-htmlelement-interface">
+<link rel="help" href="https://drafts.csswg.org/cssom-view/#scrolling-events">
+<div id=log></div>
+<div id="container">
+</div>
+<script>
+
+function setupTarget() {
+  var container = document.getElementById("container");
+  container.innerHTML = "";
+
+  var target = document.createElement("div");
+  var overflowing_child = document.createElement("div");
+
+  target.style = "overflow:scroll; height: 100px; width: 100px; scrollbar-width: none";
+  overflowing_child.style = "height: 200px; width: 200px;";
+  target.appendChild(overflowing_child);
+  container.appendChild(target);
+  return target;
+}
+
+promise_test(async (t) => {
+  var target = setupTarget();
+
+  assert_equals(target.scrollTop, 0);
+  var promiseForScrollTop = waitForEvent("scroll", t, target);
+  target.scrollTop = 10;
+  await promiseForScrollTop;
+  assert_equals(target.scrollTop, 10);
+
+  assert_equals(target.scrollLeft, 0);
+  var promiseForScrollLeft = waitForEvent("scroll", t, target);
+  target.scrollLeft = 10;
+  await promiseForScrollLeft;
+  assert_equals(target.scrollLeft, 10);
+
+}, "scrollTop and scrollLeft should fire scroll event.");
+
+promise_test(async (t) => {
+  var target = setupTarget();
+
+  target.addEventListener("scroll", () => assert_unreached("Any scroll event should not be observed"));
+
+  assert_equals(target.scrollTop, 0);
+  target.scrollTop = 0;
+
+  assert_equals(target.scrollLeft, 0);
+  target.scrollLeft = 0;
+
+  // Ensure all scroll event is flushed
+  await waitForNextFrame();
+  await waitForNextFrame();
+}, "scrollTop and scrollLeft being set with the same value.");
+
+promise_test(async (t) => {
+  var target = setupTarget();
+
+  target.addEventListener("scroll", () => assert_unreached("Any scroll event should not be observed"));
+
+  target.scrollTop = -100;
+  target.scrollLeft = -100;
+
+  // Ensure all scroll event is flushed
+  await waitForNextFrame();
+  await waitForNextFrame();
+}, "scrollTop and scrollLeft being set with invalid scroll offset.");
+
+promise_test(async (t) => {
+  var target = setupTarget();
+
+  assert_equals(target.scrollTop, 0);
+  var promiseForScrollTop = waitForEvent("scroll", t, target);
+  target.scrollTop = 1000;
+  await promiseForScrollTop;
+  assert_equals(target.scrollTop, 100);
+
+  assert_equals(target.scrollLeft, 0);
+  var promiseForScrollLeft = waitForEvent("scroll", t, target);
+  target.scrollLeft = 1000;
+  await promiseForScrollLeft;
+  assert_equals(target.scrollLeft, 100);
+
+  target.addEventListener("scroll", () => assert_unreached("Any scroll event should not be observed"));
+
+  target.scrollTop = 1000;
+  target.scrollLeft = 1000;
+
+  // Ensure all scroll event is flushed
+  await waitForNextFrame();
+  await waitForNextFrame();
+}, "scrollTop and scrollLeft when scrolling above maximum offset.");
+
+</script>

tests/wpt/tests/dom/events/scrolling/scroll-event-fired-to-iframe.html

@@ -0,0 +1,116 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Scroll event should behave correctly for Element.ScrollX and Element.ScrollLeft</title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script src="scroll_support.js"></script>
+<link rel="author" title="Jo Steven Novaryo" href="mailto:jo.steven.novaryo@huawei.com">
+<link rel="help" href="https://drafts.csswg.org/cssom-view/#extensions-to-the-window-interface">
+<link rel="help" href="https://drafts.csswg.org/cssom-view/#scrolling-events">
+<div id=log></div>
+<div id="container">
+</div>
+<script>
+
+
+function promiseForFrameLoad(frame) {
+  return new Promise(async (resolve) => {
+    frame.addEventListener("load", () => resolve(frame), { once: true });
+  });
+}
+
+async function promiseForSetupTargetFrame() {
+  const target = document.createElement("iframe");
+  target.src = "../resources/large-dimension-document.sub.html";
+  target.width = "200";
+  target.height = "200";
+
+  var container = document.getElementById("container");
+  container.innerHTML = "";
+  container.appendChild(target);
+
+  return promiseForFrameLoad(target);
+}
+
+promise_test(async (t) => {
+  var frame = await promiseForSetupTargetFrame();
+  var target = frame.contentWindow;
+
+  assert_equals(target.scrollX, 0);
+  var promiseForScrollX = waitForEvent("scroll", t, target);
+  target.scrollTo({left: 10});
+  await promiseForScrollX;
+  assert_equals(target.scrollX, 10);
+
+  assert_equals(target.scrollY, 0);
+  var promiseForScrollY = waitForEvent("scroll", t, target);
+  target.scrollTo({top: 10});
+  await promiseForScrollY;
+  assert_equals(target.scrollY, 10);
+
+}, "scrollX and scrollY should fire scroll event.");
+
+promise_test(async (t) => {
+  var frame = await promiseForSetupTargetFrame();
+  var target = frame.contentWindow;
+
+  target.addEventListener("scroll", () => assert_unreached("Any scroll event should not be observed"));
+
+  assert_equals(target.scrollX, 0);
+  target.scrollTo({left: 0});
+
+  assert_equals(target.scrollY, 0);
+  target.scrollTo({top: 0});
+
+  // Ensure all scroll event is flushed
+  await waitForNextFrame();
+  await waitForNextFrame();
+}, "scrollX and scrollY being set with the same value.");
+
+promise_test(async (t) => {
+  var frame = await promiseForSetupTargetFrame();
+  var target = frame.contentWindow;
+
+  target.addEventListener("scroll", () => assert_unreached("Any scroll event should not be observed"));
+
+  target.scrollTo({left: -100});
+  target.scrollTo({top: -100});
+
+  // Ensure all scroll event is flushed
+  await waitForNextFrame();
+  await waitForNextFrame();
+}, "scrollX and scrollY being set with invalid scroll Scroll.");
+
+promise_test(async (t) => {
+  var frame = await promiseForSetupTargetFrame();
+  var target = frame.contentWindow;
+  var frameDocEl = frame.contentDocument.documentElement;
+
+  assert_equals(target.scrollX, 0);
+  var promiseForScrollX = waitForEvent("scroll", t, target);
+  target.scrollTo({left: frameDocEl.scrollWidth});
+  await promiseForScrollX;
+  assert_greater_than(target.scrollX, 0);
+
+  assert_equals(target.scrollY, 0);
+  var promiseForScrollY = waitForEvent("scroll", t, target);
+  target.scrollTo({
+    top: frameDocEl.scrollHeight,
+    left: target.scrollX
+  });
+  await promiseForScrollY;
+  assert_greater_than(target.scrollY, 0);
+
+  target.addEventListener("scroll", () => assert_unreached("Any scroll event should not be observed"));
+
+  target.scrollTo({
+    left: target.scrollX + 10,
+    top: target.scrollY + 10,
+  });
+
+  // Ensure all scroll event is flushed
+  await waitForNextFrame();
+  await waitForNextFrame();
+}, "scrollX and scrollY when scrolling above maximum Scroll.");
+
+</script>