From d29ad5b9c3fb5aa7b46ce0158339fc03c472cef4 Mon Sep 17 00:00:00 2001
From: Oriol Brufau <obrufau@igalia.com>
Date: Fri, 26 Sep 2025 19:14:45 +0200
Subject: [PATCH] compositing: Don't reset scroll positions when receiving a
 new frame tree (#39515)

In the past, all scroll offsets were reset when navigating between
pages,
and layouts probably didn't preserve their scroll positions between back
and forward operations. Whatever the reason, the compositor was still
trying to
reset these positions right after a load.

This is an issue, because a page can request a scroll during a load, and
this
scroll might be requested from `<iframe>` content. In that case, the
scroll
positions should be the ones that layout reflects and not cleared from
the
Compositor. This reset operation had the potential to stomp on scroll
positions
set during load.

It's quite likely that the Compositor shouldn't be trying to reset them
at all,
so just stop doing that. This removes a source of intermittency when
running
tests that set scroll positions.

Testing: Once #39475 lands after this, its test should always pass.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>
---
 components/compositing/webview_renderer.rs | 30 +---------------------
 1 file changed, 1 insertion(+), 29 deletions(-)

diff --git a/components/compositing/webview_renderer.rs b/components/compositing/webview_renderer.rs
index 79346555e4b..b5f73dc760d 100644
--- a/components/compositing/webview_renderer.rs
+++ b/components/compositing/webview_renderer.rs
@@ -21,7 +21,7 @@ use embedder_traits::{
 use euclid::{Point2D, Scale, Vector2D};
 use log::{debug, warn};
 use malloc_size_of::MallocSizeOf;
-use rustc_hash::{FxHashMap, FxHashSet};
+use rustc_hash::FxHashMap;
 use servo_geometry::DeviceIndependentPixel;
 use style_traits::{CSSPixel, PinchZoomFactor};
 use webrender_api::units::{DeviceIntPoint, DevicePixel, DevicePoint, DeviceRect, LayoutVector2D};
@@ -180,8 +180,6 @@ impl WebViewRenderer {
         }
 
         self.set_frame_tree_on_pipeline_details(frame_tree, None);
-        self.reset_scroll_tree_for_unattached_pipelines(frame_tree);
-        self.send_scroll_positions_to_layout_for_pipeline(pipeline_id);
     }
 
     pub(crate) fn send_scroll_positions_to_layout_for_pipeline(&self, pipeline_id: PipelineId) {
@@ -219,32 +217,6 @@ impl WebViewRenderer {
         }
     }
 
-    pub(crate) fn reset_scroll_tree_for_unattached_pipelines(
-        &mut self,
-        frame_tree: &SendableFrameTree,
-    ) {
-        // TODO(mrobinson): Eventually this can selectively preserve the scroll trees
-        // state for some unattached pipelines in order to preserve scroll position when
-        // navigating backward and forward.
-        fn collect_pipelines(
-            pipelines: &mut FxHashSet<PipelineId>,
-            frame_tree: &SendableFrameTree,
-        ) {
-            pipelines.insert(frame_tree.pipeline.id);
-            for kid in &frame_tree.children {
-                collect_pipelines(pipelines, kid);
-            }
-        }
-
-        let mut attached_pipelines: FxHashSet<PipelineId> = FxHashSet::default();
-        collect_pipelines(&mut attached_pipelines, frame_tree);
-
-        self.pipelines
-            .iter_mut()
-            .filter(|(id, _)| !attached_pipelines.contains(id))
-            .for_each(|(_, details)| details.scroll_tree.reset_all_scroll_offsets());
-    }
-
     /// Sets or unsets the animations-running flag for the given pipeline. Returns
     /// true if the pipeline has started animating.
     pub(crate) fn change_pipeline_running_animations_state(