From b5482e34c017c64fae6f5c1db41880d826546a46 Mon Sep 17 00:00:00 2001
From: Mukilan Thiyagarajan <mukilan@igalia.com>
Date: Mon, 22 Jul 2024 18:12:44 +0530
Subject: [PATCH] compositor: propagate scroll events across pipelines (#32829)

Currently we don't propagate the scroll events across pipeline and
scroll only nodes contained within the nearest pipeline that passes hit
test at a gived point. This causes iframes to "capture" the scroll and
halt the scrolling process once the iframe has been scrolled all the
way. The expected behaviour is that the parent page begins to scroll
once the iframe has been scrolled fully.

The issue is present in both desktop and mobile, but was more noticeable
on mobile, especially on the default servo.org page, because of the relative
sizes of the YouTube video's iframe wrt to the parent.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>
---
 components/compositing/compositor.rs | 36 ++++++++++++++++++----------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 2012f7fe31c..ec5a8513304 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -1808,19 +1808,31 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             ScrollLocation::Start | ScrollLocation::End => scroll_location,
         };
 
-        let hit_test_result = match self.hit_test_at_point(cursor) {
-            Some(result) => result,
-            None => return None,
-        };
+        let hit_test_results =
+            self.hit_test_at_point_with_flags_and_pipeline(cursor, HitTestFlags::FIND_ALL, None);
 
-        let pipeline_details = match self.pipeline_details.get_mut(&hit_test_result.pipeline_id) {
-            Some(details) => details,
-            None => return None,
-        };
-        pipeline_details
-            .scroll_tree
-            .scroll_node_or_ancestor(&hit_test_result.scroll_tree_node, scroll_location)
-            .map(|(external_id, offset)| (hit_test_result.pipeline_id, external_id, offset))
+        // Iterate through all hit test results, processing only the first node of each pipeline.
+        // This is needed to propagate the scroll events from a pipeline representing an iframe to
+        // its ancestor pipelines.
+        let mut previous_pipeline_id = None;
+        for CompositorHitTestResult {
+            pipeline_id,
+            scroll_tree_node,
+            ..
+        } in hit_test_results.iter()
+        {
+            if previous_pipeline_id.replace(pipeline_id) != Some(pipeline_id) {
+                let scroll_result = self
+                    .pipeline_details
+                    .get_mut(&pipeline_id)?
+                    .scroll_tree
+                    .scroll_node_or_ancestor(&scroll_tree_node, scroll_location);
+                if let Some((external_id, offset)) = scroll_result {
+                    return Some((*pipeline_id, external_id, offset));
+                }
+            }
+        }
+        None
     }
 
     /// If there are any animations running, dispatches appropriate messages to the constellation.