tor-browser

commit f24cbbb7a2d1c2fe21276360d3f2aa1eb92c6834
parent 0aec81c009fd130661ddd22885d63fd192a4623b
Author: Timothy Nikkel <tnikkel@gmail.com>
Date:   Wed, 26 Nov 2025 13:55:42 +0000

Bug 1988032. Add a pref to control async scroll of CSS anchor positioned content. r=layout-reviewers,hiro

This gives an easy way to disable in case of issues and allows toggling for testing purposes (to narrow down if a potential issue is async or main thread scrolling related).

Differential Revision: https://phabricator.services.mozilla.com/D273414

Diffstat:
M
layout/base/AnchorPositioningUtils.cpp
 | 
4
++++
M
layout/generic/ScrollContainerFrame.cpp
 | 
3
++-
M
modules/libpref/init/StaticPrefList.yaml
 | 
6
++++++

3 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/layout/base/AnchorPositioningUtils.cpp b/layout/base/AnchorPositioningUtils.cpp
@@ -9,6 +9,7 @@
 #include "ScrollContainerFrame.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/PresShell.h"
+#include "mozilla/StaticPrefs_apz.h"
 #include "mozilla/dom/Document.h"
 #include "mozilla/dom/Element.h"
 #include "nsCanvasFrame.h"
@@ -876,6 +877,9 @@ bool AnchorPositioningUtils::FitsInContainingBlock(
 
 nsIFrame* AnchorPositioningUtils::GetAnchorThatFrameScrollsWith(
     nsIFrame* aFrame) {
+  if (!StaticPrefs::apz_async_scroll_css_anchor_pos_AtStartup()) {
+    return nullptr;
+  }
   mozilla::PhysicalAxes axes = aFrame->GetAnchorPosCompensatingForScroll();
   // TODO for now we return the anchor if we are compensating in either axis.
   // This is not fully spec compliant, bug 1988034 tracks this.
diff --git a/layout/generic/ScrollContainerFrame.cpp b/layout/generic/ScrollContainerFrame.cpp
@@ -4340,7 +4340,8 @@ nsRect ScrollContainerFrame::RestrictToRootDisplayPort(
       FissionAutostart()) {
     return true;
   }
-  return aFrame->PresShell()->GetRootPresShell()->HasSeenAnchorPos();
+  return StaticPrefs::apz_async_scroll_css_anchor_pos_AtStartup() &&
+         aFrame->PresShell()->GetRootPresShell()->HasSeenAnchorPos();
 }
 
 bool ScrollContainerFrame::DecideScrollableLayerEnsureDisplayport(
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
@@ -953,6 +953,12 @@
   value: false
   mirror: always
 
+# Whether to async scroll CSS anchor pos.
+- name: apz.async_scroll_css_anchor_pos
+  type: bool
+  value: true
+  mirror: once
+
 #---------------------------------------------------------------------------
 # Prefs starting with "beacon."
 #---------------------------------------------------------------------------