diff --git a/layout/generic/ScrollAnchorContainer.cpp b/layout/generic/ScrollAnchorContainer.cpp
index 159ee4fbc93e..ecc5c6b9ce6f 100644
--- a/layout/generic/ScrollAnchorContainer.cpp
+++ b/layout/generic/ScrollAnchorContainer.cpp
@@ -7,6 +7,7 @@
#include "ScrollAnchorContainer.h"
#include "mozilla/dom/Text.h"
+#include "mozilla/ScopeExit.h"
#include "mozilla/PresShell.h"
#include "mozilla/ProfilerLabels.h"
#include "mozilla/StaticPrefs_layout.h"
@@ -45,6 +46,7 @@ ScrollAnchorContainer::ScrollAnchorContainer(ScrollFrameHelper* aScrollFrame)
mAnchorNode(nullptr),
mLastAnchorOffset(0),
mDisabled(false),
+ mAnchorMightBeSubOptimal(false),
mAnchorNodeIsDirty(true),
mApplyingAnchorAdjustment(false),
mSuppressAnchorAdjustment(false) {}
@@ -263,6 +265,8 @@ void ScrollAnchorContainer::SelectAnchor() {
ANCHOR_LOG("Skipping selection, doesn't maintain a scroll anchor.\n");
mAnchorNode = nullptr;
}
+ mAnchorMightBeSubOptimal =
+ mAnchorNode && mAnchorNode->HasAnyStateBits(NS_FRAME_HAS_DIRTY_CHILDREN);
// Update the anchor flags if needed
if (oldAnchor != mAnchorNode) {
@@ -400,6 +404,7 @@ void ScrollAnchorContainer::InvalidateAnchor(ScheduleSelection aSchedule) {
FindFor(Frame())->InvalidateAnchor();
}
mAnchorNode = nullptr;
+ mAnchorMightBeSubOptimal = false;
mAnchorNodeIsDirty = true;
mLastAnchorOffset = 0;
@@ -445,6 +450,16 @@ void ScrollAnchorContainer::ApplyAdjustments() {
ANCHOR_LOG("Anchor has moved from %d to %d.\n", mLastAnchorOffset, current);
+ auto maybeInvalidate = MakeScopeExit([&] {
+ if (mAnchorMightBeSubOptimal &&
+ StaticPrefs::layout_css_scroll_anchoring_reselect_if_suboptimal()) {
+ ANCHOR_LOG(
+ "Anchor might be suboptimal, invalidating to try finding a better "
+ "one\n");
+ InvalidateAnchor();
+ }
+ });
+
if (logicalAdjustment == 0) {
ANCHOR_LOG("Ignoring zero delta anchor adjustment for %p.\n", this);
mSuppressAnchorAdjustment = false;
diff --git a/layout/generic/ScrollAnchorContainer.h b/layout/generic/ScrollAnchorContainer.h
index 08f2a76a74ea..56b2a20241dc 100644
--- a/layout/generic/ScrollAnchorContainer.h
+++ b/layout/generic/ScrollAnchorContainer.h
@@ -167,6 +167,9 @@ class ScrollAnchorContainer final {
// layout.css.scroll-anchoring.min-adjustment-threshold.
bool mDisabled : 1;
+ // True if when we selected the current scroll anchor, there were unlaid out
+ // children that could be better anchor nodes after layout.
+ bool mAnchorMightBeSubOptimal : 1;
// True if we should recalculate our anchor node at the next chance
bool mAnchorNodeIsDirty : 1;
// True if we are applying a scroll anchor adjustment
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index aee083d6cef5..02c0e79c68f2 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -7596,6 +7596,14 @@
value: false
mirror: always
+# Pref to control whether we reselect scroll anchors if sub-optimal
+#
+# See https://github.com/w3c/csswg-drafts/issues/6787
+- name: layout.css.scroll-anchoring.reselect-if-suboptimal
+ type: bool
+ value: true
+ mirror: always
+
# Are shared memory User Agent style sheets enabled?
- name: layout.css.shared-memory-ua-sheets.enabled
type: bool
diff --git a/testing/web-platform/tests/css/css-scroll-anchoring/dirty-contents-reselect-anchor.tentative.html b/testing/web-platform/tests/css/css-scroll-anchoring/dirty-contents-reselect-anchor.tentative.html
new file mode 100644
index 000000000000..41adf53a0f92
--- /dev/null
+++ b/testing/web-platform/tests/css/css-scroll-anchoring/dirty-contents-reselect-anchor.tentative.html
@@ -0,0 +1,54 @@
+
+
+
+
+
+
+
+
+
+