Bug 1305957 part 5 - Add implementation of scroll anchor selection and invalidation. r=hiro,dbaron,dholbert

This commit implements candidate selection for a scroll frame using a frame tree
traversal. It roughly tries to follow the algorithm given in the scroll
anchoring draft specification, adapted to operate on the frame tree [1].

Some details, such as not selecting an anchor if the user hasn't scrolled are
not currently in the specification but will be to match Blink's implementation.

Once a scroll anchor has been selected, we maintain a bit on it and its ancestor
frame's states. This is used in a later commit to detect changes to position
during a reflow so the scroll frame can perform an adjustment.

A scroll anchor will be invalidated when the user scrolls the frame or the
scroll anchor is destroyed. Later commits will add logic to drive selection and
invalidation appropriately.

[1] https://drafts.csswg.org/css-scroll-anchoring/#anchor-node-selection

Differential Revision: https://phabricator.services.mozilla.com/D13268

--HG--
extra : rebase_source : e05ba48662aafef5957322def33ddc5d93f3ca5a
extra : source : bdb766faa8679386bf4e9740781262ea4bb36544

layout/base/nsLayoutUtils.cpp

@@ -1490,11 +1490,11 @@ bool nsLayoutUtils::IsAncestorFrameCrossDoc(const nsIFrame* aAncestorFrame,
 }
 
 // static
-bool nsLayoutUtils::IsProperAncestorFrame(nsIFrame* aAncestorFrame,
-                                          nsIFrame* aFrame,
-                                          nsIFrame* aCommonAncestor) {
+bool nsLayoutUtils::IsProperAncestorFrame(const nsIFrame* aAncestorFrame,
+                                          const nsIFrame* aFrame,
+                                          const nsIFrame* aCommonAncestor) {
   if (aFrame == aAncestorFrame) return false;
-  for (nsIFrame* f = aFrame; f != aCommonAncestor; f = f->GetParent()) {
+  for (const nsIFrame* f = aFrame; f != aCommonAncestor; f = f->GetParent()) {
     if (f == aAncestorFrame) return true;
   }
   return aCommonAncestor == aAncestorFrame;

layout/base/nsLayoutUtils.h

@@ -528,8 +528,9 @@ class nsLayoutUtils {
    * aAncestorFrame. If non-null, this can bound the search and speed up
    * the function
    */
-  static bool IsProperAncestorFrame(nsIFrame* aAncestorFrame, nsIFrame* aFrame,
-                                    nsIFrame* aCommonAncestor = nullptr);
+  static bool IsProperAncestorFrame(const nsIFrame* aAncestorFrame,
+                                    const nsIFrame* aFrame,
+                                    const nsIFrame* aCommonAncestor = nullptr);
 
   /**
    * Like IsProperAncestorFrame, but looks across document boundaries.

layout/generic/ScrollAnchorContainer.cpp

@@ -6,6 +6,7 @@
 
 #include "ScrollAnchorContainer.h"
 
+#include "mozilla/StaticPrefs.h"
 #include "nsGfxScrollFrame.h"
 #include "nsLayoutUtils.h"
 
@@ -16,7 +17,10 @@ namespace mozilla {
 namespace layout {
 
 ScrollAnchorContainer::ScrollAnchorContainer(ScrollFrameHelper* aScrollFrame)
-    : mScrollFrame(aScrollFrame) {}
+    : mScrollFrame(aScrollFrame),
+      mAnchorNode(nullptr),
+      mLastAnchorPos(0, 0),
+      mAnchorNodeIsDirty(true) {}
 
 ScrollAnchorContainer::~ScrollAnchorContainer() {}
 
@@ -40,5 +44,393 @@ nsIScrollableFrame* ScrollAnchorContainer::ScrollableFrame() const {
   return Frame()->GetScrollTargetFrame();
 }
 
+/**
+ * Set the appropriate frame flags for a frame that has become or is no longer
+ * an anchor node.
+ */
+static void SetAnchorFlags(const nsIFrame* aScrolledFrame,
+                           nsIFrame* aAnchorNode, bool aInScrollAnchorChain) {
+  nsIFrame* frame = aAnchorNode;
+  while (frame && frame != aScrolledFrame) {
+    MOZ_ASSERT(
+        frame == aAnchorNode || !frame->IsScrollFrame(),
+        "We shouldn't select an anchor node inside a nested scroll frame.");
+
+    frame->SetInScrollAnchorChain(aInScrollAnchorChain);
+    frame = frame->GetParent();
+  }
+  MOZ_ASSERT(frame,
+             "The anchor node should be a descendant of the scroll frame");
+}
+
+/**
+ * Compute the scrollable overflow rect [1] of aCandidate relative to
+ * aScrollFrame with all transforms applied. The specification is also
+ * ambiguous about what can be selected as a scroll anchor, which makes
+ * the scroll anchoring bounding rect partially undefined [2]. This code
+ * attempts to match the implementation in Blink.
+ *
+ * [1]
+ * https://drafts.csswg.org/css-scroll-anchoring-1/#scroll-anchoring-bounding-rect
+ * [2] https://github.com/w3c/csswg-drafts/issues/3478
+ */
+static nsRect FindScrollAnchoringBoundingRect(const nsIFrame* aScrollFrame,
+                                              nsIFrame* aCandidate) {
+  MOZ_ASSERT(nsLayoutUtils::IsProperAncestorFrame(aScrollFrame, aCandidate));
+  if (aCandidate->GetContent()->IsText()) {
+    nsRect bounding;
+    for (nsIFrame* continuation = aCandidate->FirstContinuation(); continuation;
+         continuation = continuation->GetNextContinuation()) {
+      nsRect localRect =
+          continuation->GetScrollableOverflowRectRelativeToSelf();
+      nsRect transformed = nsLayoutUtils::TransformFrameRectToAncestor(
+          continuation, localRect, aScrollFrame);
+      bounding = bounding.Union(transformed);
+    }
+    return bounding;
+  }
+
+  nsRect localRect = aCandidate->GetScrollableOverflowRectRelativeToSelf();
+  nsRect transformed = nsLayoutUtils::TransformFrameRectToAncestor(
+      aCandidate, localRect, aScrollFrame);
+  return transformed;
+}
+
+void ScrollAnchorContainer::SelectAnchor() {
+  MOZ_ASSERT(mScrollFrame->mScrolledFrame);
+  MOZ_ASSERT(mAnchorNodeIsDirty);
+
+  if (!StaticPrefs::layout_css_scroll_anchoring_enabled()) {
+    return;
+  }
+
+  ANCHOR_LOG("Selecting anchor for %p with scroll-port [%d %d x %d %d].\n",
+             this, mScrollFrame->mScrollPort.x, mScrollFrame->mScrollPort.y,
+             mScrollFrame->mScrollPort.width, mScrollFrame->mScrollPort.height);
+
+  const nsStyleDisplay* disp = Frame()->StyleDisplay();
+
+  // Don't select a scroll anchor if the scroll frame has `overflow-anchor:
+  // none`.
+  bool overflowAnchor =
+      disp->mOverflowAnchor == mozilla::StyleOverflowAnchor::Auto;
+
+  // Or if the scroll frame has not been scrolled from the logical origin. This
+  // is not in the specification [1], but Blink does this.
+  //
+  // [1] https://github.com/w3c/csswg-drafts/issues/3319
+  bool isScrolled = mScrollFrame->GetLogicalScrollPosition() != nsPoint();
+
+  // Or if there is perspective that could affect the scrollable overflow rect
+  // for descendant frames. This is not in the specification as Blink doesn't
+  // share this behavior with perspective [1].
+  //
+  // [1] https://github.com/w3c/csswg-drafts/issues/3322
+  bool hasPerspective = Frame()->ChildrenHavePerspective();
+
+  // Select a new scroll anchor
+  nsIFrame* oldAnchor = mAnchorNode;
+  if (overflowAnchor && isScrolled && !hasPerspective) {
+    ANCHOR_LOG("Beginning candidate selection.\n");
+    mAnchorNode = FindAnchorIn(mScrollFrame->mScrolledFrame);
+  } else {
+    if (!overflowAnchor) {
+      ANCHOR_LOG("Skipping candidate selection for `overflow-anchor: none`\n");
+    }
+    if (!isScrolled) {
+      ANCHOR_LOG("Skipping candidate selection for not being scrolled\n");
+    }
+    if (hasPerspective) {
+      ANCHOR_LOG(
+          "Skipping candidate selection for scroll frame with perspective\n");
+    }
+    mAnchorNode = nullptr;
+  }
+
+  // Update the anchor flags if needed
+  if (oldAnchor != mAnchorNode) {
+    ANCHOR_LOG("Anchor node has changed from (%p) to (%p).\n", oldAnchor,
+               mAnchorNode);
+
+    // Unset all flags for the old scroll anchor
+    if (oldAnchor) {
+      SetAnchorFlags(mScrollFrame->mScrolledFrame, oldAnchor, false);
+    }
+
+    // Set all flags for the new scroll anchor
+    if (mAnchorNode) {
+      // Anchor selection will never select a descendant of a different scroll
+      // frame, so we can set flags without conflicting with other scroll
+      // anchor containers.
+      SetAnchorFlags(mScrollFrame->mScrolledFrame, mAnchorNode, true);
+    }
+  } else {
+    ANCHOR_LOG("Anchor node has remained (%p).\n", mAnchorNode);
+  }
+
+  // Calculate the position to use for scroll adjustments
+  if (mAnchorNode) {
+    mLastAnchorPos =
+        FindScrollAnchoringBoundingRect(Frame(), mAnchorNode).TopLeft();
+    ANCHOR_LOG("Using last anchor position = [%d, %d].\n", mLastAnchorPos.x,
+               mLastAnchorPos.y);
+  } else {
+    mLastAnchorPos = nsPoint();
+  }
+
+  mAnchorNodeIsDirty = false;
+}
+
+void ScrollAnchorContainer::UserScrolled() { InvalidateAnchor(); }
+
+void ScrollAnchorContainer::InvalidateAnchor() {
+  if (!StaticPrefs::layout_css_scroll_anchoring_enabled()) {
+    return;
+  }
+
+  ANCHOR_LOG("Invalidating scroll anchor %p for %p.\n", mAnchorNode, this);
+
+  if (mAnchorNode) {
+    SetAnchorFlags(mScrollFrame->mScrolledFrame, mAnchorNode, false);
+  }
+  mAnchorNode = nullptr;
+  mAnchorNodeIsDirty = true;
+  mLastAnchorPos = nsPoint();
+}
+
+void ScrollAnchorContainer::Destroy() {
+  if (mAnchorNode) {
+    SetAnchorFlags(mScrollFrame->mScrolledFrame, mAnchorNode, false);
+  }
+  mAnchorNode = nullptr;
+  mAnchorNodeIsDirty = false;
+  mLastAnchorPos = nsPoint();
+}
+
+ScrollAnchorContainer::ExamineResult
+ScrollAnchorContainer::ExamineAnchorCandidate(nsIFrame* aFrame) const {
+#ifdef DEBUG_FRAME_DUMP
+  nsCString tag = aFrame->ListTag();
+  ANCHOR_LOG("\tVisiting frame=%s (%p).\n", tag.get(), aFrame);
+#else
+  ANCHOR_LOG("\t\tVisiting frame=%p.\n", aFrame);
+#endif
+
+  // Check if the author has opted out of scroll anchoring for this frame
+  // and its descendants.
+  const nsStyleDisplay* disp = aFrame->StyleDisplay();
+  if (disp->mOverflowAnchor == mozilla::StyleOverflowAnchor::None) {
+    ANCHOR_LOG("\t\tExcluding `overflow-anchor: none`.\n");
+    return ExamineResult::Exclude;
+  }
+
+  // Sticky positioned elements can move with the scroll frame, making them
+  // unsuitable scroll anchors. This isn't in the specification yet [1], but
+  // matches Blink's implementation.
+  //
+  // [1] https://github.com/w3c/csswg-drafts/issues/3319
+  if (aFrame->IsStickyPositioned()) {
+    ANCHOR_LOG("\t\tExcluding `position: sticky`.\n");
+    return ExamineResult::Exclude;
+  }
+
+  // The frame for a <br> element has a non-zero area, but Blink treats them
+  // as if they have no area, so exclude them specially.
+  if (aFrame->IsBrFrame()) {
+    ANCHOR_LOG("\t\tExcluding <br>.\n");
+    return ExamineResult::Exclude;
+  }
+
+  // Exclude frames that aren't accessible to content.
+  bool isChrome =
+      aFrame->GetContent() && aFrame->GetContent()->ChromeOnlyAccess();
+  bool isPseudo = aFrame->Style()->IsPseudoElement();
+  if (isChrome && !isPseudo) {
+    ANCHOR_LOG("\t\tExcluding chrome only content.\n");
+    return ExamineResult::Exclude;
+  }
+
+  // See if this frame could have its own anchor node. We could check
+  // IsScrollFrame(), but that would miss nsListControlFrame which is not a
+  // scroll frame, but still inherits from nsHTMLScrollFrame.
+  nsIScrollableFrame* scrollable = do_QueryFrame(aFrame);
+
+  // We don't allow scroll anchors to be selected inside of scrollable frames as
+  // it's not clear how an anchor adjustment should apply to multiple scrollable
+  // frames. Blink allows this to happen, but they're not sure why [1].
+  //
+  // We also don't allow scroll anchors to be selected inside of SVG as it uses
+  // a different layout model than CSS, and the specification doesn't say it
+  // should apply.
+  //
+  // [1] https://github.com/w3c/csswg-drafts/issues/3477
+  bool canDescend = !scrollable && !aFrame->IsSVGOuterSVGFrame();
+
+  // Check what kind of frame this is
+  bool isBlockOutside = aFrame->IsBlockOutside();
+  bool isText = aFrame->GetContent()->IsText();
+  bool isAnonBox = aFrame->Style()->IsAnonBox() && !isText;
+  bool isInlineOutside = aFrame->IsInlineOutside() && !isText;
+  bool isContinuation = !!aFrame->GetPrevContinuation();
+
+  // If the frame is anonymous or inline-outside, search its descendants for a
+  // scroll anchor.
+  if ((isAnonBox || isInlineOutside) && canDescend) {
+    ANCHOR_LOG(
+        "\t\tSearching descendants of anon or inline box (a=%d, i=%d).\n",
+        isAnonBox, isInlineOutside);
+    return ExamineResult::PassThrough;
+  }
+
+  // If the frame is not block-outside or a text node then exclude it.
+  if (!isBlockOutside && !isText) {
+    ANCHOR_LOG("\t\tExcluding non block-outside or text node (b=%d, t=%d).\n",
+               isBlockOutside, isText);
+    return ExamineResult::Exclude;
+  }
+
+  // Find the scroll anchoring bounding rect.
+  nsRect rect = FindScrollAnchoringBoundingRect(Frame(), aFrame);
+  ANCHOR_LOG("\t\trect = [%d %d x %d %d].\n", rect.x, rect.y, rect.width,
+             rect.height);
+
+  // Check if this frame is visible in the scroll port. This will exclude rects
+  // with zero sized area. The specification is ambiguous about this [1], but
+  // this matches Blink's implementation.
+  //
+  // [1] https://github.com/w3c/csswg-drafts/issues/3483
+  nsRect visibleRect;
+  if (!visibleRect.IntersectRect(rect, mScrollFrame->mScrollPort)) {
+    return ExamineResult::Exclude;
+  }
+
+  // At this point, if canDescend is true, we should only have visible
+  // non-anonymous frames that are either:
+  //   1. block-outside
+  //   2. text nodes
+  //
+  // It's not clear what the scroll anchoring bounding rect of elements that are
+  // block-outside should be when they are fragmented. For text nodes that are
+  // fragmented, it's specified that we need to consider the union of its line
+  // boxes.
+  //
+  // So for text nodes we handle them by including the union of line boxes in
+  // the bounding rect of the primary frame, and not selecting any
+  // continuations.
+  //
+  // For block-outside elements we choose to consider the bounding rect of each
+  // frame individually, allowing ourselves to descend into any frame, but only
+  // selecting a frame if it's not a continuation.
+  if (canDescend && isContinuation) {
+    ANCHOR_LOG("\t\tSearching descendants of a continuation.\n");
+    return ExamineResult::PassThrough;
+  }
+
+  // If this frame is fully visible, then select it as the scroll anchor.
+  if (visibleRect.IsEqualEdges(rect)) {
+    ANCHOR_LOG("\t\tFully visible, taking.\n");
+    return ExamineResult::Accept;
+  }
+
+  // If we can't descend into this frame, then select it as the scroll anchor.
+  if (!canDescend) {
+    ANCHOR_LOG("\t\tIntersects a frame that we can't descend into, taking.\n");
+    return ExamineResult::Accept;
+  }
+
+  // It must be partially visible and we can descend into this frame. Examine
+  // its children for a better scroll anchor or fall back to this one.
+  ANCHOR_LOG("\t\tIntersects valid candidate, checking descendants.\n");
+  return ExamineResult::Traverse;
+}
+
+nsIFrame* ScrollAnchorContainer::FindAnchorIn(nsIFrame* aFrame) const {
+  // Visit the child lists of this frame
+  for (nsIFrame::ChildListIterator lists(aFrame); !lists.IsDone();
+       lists.Next()) {
+    // Skip child lists that contain out-of-flow frames, we'll visit them by
+    // following placeholders in the in-flow lists so that we visit these
+    // frames in DOM order.
+    // XXX do we actually need to exclude kOverflowOutOfFlowList too?
+    if (lists.CurrentID() == FrameChildListID::kAbsoluteList ||
+        lists.CurrentID() == FrameChildListID::kFixedList ||
+        lists.CurrentID() == FrameChildListID::kFloatList ||
+        lists.CurrentID() == FrameChildListID::kOverflowOutOfFlowList) {
+      continue;
+    }
+
+    // Search the child list, and return if we selected an anchor
+    if (nsIFrame* anchor = FindAnchorInList(lists.CurrentList())) {
+      return anchor;
+    }
+  }
+
+  // The spec requires us to do an extra pass to visit absolutely positioned
+  // frames a second time after all the children of their containing block have
+  // been visited.
+  //
+  // It's not clear why this is needed [1], but it matches Blink's
+  // implementation, and is needed for a WPT test.
+  //
+  // [1] https://github.com/w3c/csswg-drafts/issues/3465
+  const nsFrameList& absPosList =
+      aFrame->GetChildList(FrameChildListID::kAbsoluteList);
+  if (nsIFrame* anchor = FindAnchorInList(absPosList)) {
+    return anchor;
+  }
+
+  return nullptr;
+}
+
+nsIFrame* ScrollAnchorContainer::FindAnchorInList(
+    const nsFrameList& aFrameList) const {
+  for (nsIFrame* child : aFrameList) {
+    // If this is a placeholder, try to follow it to the out of flow frame.
+    nsIFrame* realFrame = nsPlaceholderFrame::GetRealFrameFor(child);
+    if (child != realFrame) {
+      // If the out of flow frame is not a descendant of our scroll frame,
+      // then it must have a different containing block and cannot be an
+      // anchor node.
+      if (!nsLayoutUtils::IsProperAncestorFrame(Frame(), realFrame)) {
+        ANCHOR_LOG(
+            "\t\tSkipping out of flow frame that is not a descendant of the "
+            "scroll frame.\n");
+        continue;
+      }
+      ANCHOR_LOG("\t\tFollowing placeholder to out of flow frame.\n");
+      child = realFrame;
+    }
+
+    // Perform the candidate examination algorithm
+    ExamineResult examine = ExamineAnchorCandidate(child);
+
+    // See the comment before the definition of `ExamineResult` in
+    // `ScrollAnchorContainer.h` for an explanation of this behavior.
+    switch (examine) {
+      case ExamineResult::Exclude: {
+        continue;
+      }
+      case ExamineResult::PassThrough: {
+        nsIFrame* candidate = FindAnchorIn(child);
+        if (!candidate) {
+          continue;
+        }
+        return candidate;
+      }
+      case ExamineResult::Traverse: {
+        nsIFrame* candidate = FindAnchorIn(child);
+        if (!candidate) {
+          return child;
+        }
+        return candidate;
+      }
+      case ExamineResult::Accept: {
+        return child;
+      }
+    }
+  }
+  return nullptr;
+}
+
 }  // namespace layout
 }  // namespace mozilla

layout/generic/ScrollAnchorContainer.h

@@ -7,6 +7,9 @@
 #ifndef mozilla_layout_ScrollAnchorContainer_h_
 #define mozilla_layout_ScrollAnchorContainer_h_
 
+#include "nsPoint.h"
+
+class nsIFrame;
 namespace mozilla {
 class ScrollFrameHelper;
 }  // namespace mozilla
@@ -32,6 +35,12 @@ class ScrollAnchorContainer final {
    */
   static ScrollAnchorContainer* FindFor(nsIFrame* aFrame);
 
+  /**
+   * Returns the frame that is the selected anchor node or null if no anchor
+   * is selected.
+   */
+  nsIFrame* AnchorNode() const { return mAnchorNode; }
+
   /**
    * Returns the frame that owns this scroll anchor container. This is always
    * non-null.
@@ -44,9 +53,80 @@ class ScrollAnchorContainer final {
    */
   nsIScrollableFrame* ScrollableFrame() const;
 
+  /**
+   * Find a suitable anchor node among the descendants of the scrollable frame.
+   * This should only be called after the scroll anchor has been invalidated.
+   */
+  void SelectAnchor();
+
+  /**
+   * Notify the scroll anchor container that its scroll frame has been
+   * scrolled by a user and should invalidate itself.
+   */
+  void UserScrolled();
+
+  /**
+   * Notify this scroll anchor container that its anchor node should be
+   * invalidated and recomputed at the next available opportunity.
+   */
+  void InvalidateAnchor();
+
+  /**
+   * Notify this scroll anchor container that it will be destroyed along with
+   * its parent frame.
+   */
+  void Destroy();
+
  private:
+  // Represents an assessment of a frame's suitability as a scroll anchor,
+  // from the scroll-anchoring spec's "candidate examination algorithm":
+  // https://drafts.csswg.org/css-scroll-anchoring-1/#candidate-examination
+  enum class ExamineResult {
+    // The frame is an excluded subtree or fully clipped and should be ignored.
+    // This corresponds with step 1 in the algorithm.
+    Exclude,
+    // This frame is an anonymous or inline box and its descendants should be
+    // searched to find an anchor node. If none are found, then continue
+    // searching. This is implied by the prologue of the algorithm, and
+    // should be made explicit in the spec [1].
+    //
+    // [1] https://github.com/w3c/csswg-drafts/issues/3489
+    PassThrough,
+    // The frame is partially visible and its descendants should be searched to
+    // find an anchor node. If none are found then this frame should be
+    // selected. This corresponds with step 3 in the algorithm.
+    Traverse,
+    // The frame is fully visible and should be selected as an anchor node. This
+    // corresponds with step 2 in the algorithm.
+    Accept,
+  };
+
+  ExamineResult ExamineAnchorCandidate(nsIFrame* aPrimaryFrame) const;
+
+  // Search a frame's children to find an anchor node. Returns the frame for a
+  // valid anchor node, if one was found in the frames descendants, or null
+  // otherwise.
+  nsIFrame* FindAnchorIn(nsIFrame* aFrame) const;
+
+  // Search a child list to find an anchor node. Returns the frame for a valid
+  // anchor node, if one was found in this child list, or null otherwise.
+  nsIFrame* FindAnchorInList(const nsFrameList& aFrameList) const;
+
   // The owner of this scroll anchor container
   ScrollFrameHelper* mScrollFrame;
+
+  // The anchor node that we will scroll to keep in the same relative position
+  // after reflows. This may be null if we were not able to select a valid
+  // scroll anchor
+  nsIFrame* mAnchorNode;
+
+  // The last position of the scroll anchor node relative to the scrollable
+  // frame. This is used for calculating the distance to scroll to keep the
+  // anchor node in the same relative position
+  nsPoint mLastAnchorPos;
+
+  // True if we should recalculate our anchor node at the next chance
+  bool mAnchorNodeIsDirty : 1;
 };
 
 }  // namespace layout

layout/generic/nsFrame.cpp

@@ -111,6 +111,7 @@
 #include "mozilla/dom/TouchEvent.h"
 #include "mozilla/gfx/Tools.h"
 #include "mozilla/layers/WebRenderUserData.h"
+#include "mozilla/layout/ScrollAnchorContainer.h"
 #include "nsPrintfCString.h"
 #include "ActiveLayerTracker.h"
 
@@ -727,6 +728,11 @@ void nsFrame::DestroyFrom(nsIFrame* aDestructRoot,
     ActiveLayerTracker::TransferActivityToContent(this, mContent);
   }
 
+  ScrollAnchorContainer* anchor = nullptr;
+  if (IsScrollAnchor(&anchor)) {
+    anchor->InvalidateAnchor();
+  }
+
   if (HasCSSAnimations() || HasCSSTransitions() ||
       EffectSet::GetEffectSet(this)) {
     // If no new frame for this element is created by the end of the
@@ -9143,6 +9149,28 @@ void nsIFrame::ComputePreserve3DChildrenOverflow(
   }
 }
 
+bool nsIFrame::IsScrollAnchor(ScrollAnchorContainer** aOutContainer) {
+  if (!mInScrollAnchorChain) {
+    return false;
+  }
+
+  ScrollAnchorContainer* container = ScrollAnchorContainer::FindFor(this);
+  if (container->AnchorNode() != this) {
+    return false;
+  }
+
+  if (aOutContainer) {
+    *aOutContainer = container;
+  }
+  return true;
+}
+
+bool nsIFrame::IsInScrollAnchorChain() const { return mInScrollAnchorChain; }
+
+void nsIFrame::SetInScrollAnchorChain(bool aInChain) {
+  mInScrollAnchorChain = aInChain;
+}
+
 uint32_t nsIFrame::GetDepthInFrameTree() const {
   uint32_t result = 0;
   for (nsContainerFrame* ancestor = GetParent(); ancestor;

layout/generic/nsGfxScrollFrame.cpp

@@ -2741,6 +2741,7 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
   }
 
   ScrollVisual();
+  mAnchor.UserScrolled();
 
   bool schedulePaint = true;
   if (nsLayoutUtils::AsyncPanZoomEnabled(mOuter) &&
@@ -4711,6 +4712,8 @@ void ScrollFrameHelper::AppendAnonymousContentTo(
 }
 
 void ScrollFrameHelper::Destroy(PostDestroyData& aPostDestroyData) {
+  mAnchor.Destroy();
+
   if (mScrollbarActivity) {
     mScrollbarActivity->Destroy();
     mScrollbarActivity = nullptr;

layout/generic/nsIFrame.h

@@ -115,6 +115,10 @@ class Layer;
 class LayerManager;
 }  // namespace layers
 
+namespace layout {
+class ScrollAnchorContainer;
+}  // namespace layout
+
 namespace dom {
 class Selection;
 }  // namespace dom
@@ -566,7 +570,8 @@ class nsIFrame : public nsQueryFrame {
         mIsPrimaryFrame(false),
         mMayHaveTransformAnimation(false),
         mMayHaveOpacityAnimation(false),
-        mAllDescendantsAreInvisible(false) {
+        mAllDescendantsAreInvisible(false),
+        mInScrollAnchorChain(false) {
     mozilla::PodZero(&mOverflow);
   }
 
@@ -1846,6 +1851,24 @@ class nsIFrame : public nsQueryFrame {
 
   void RecomputePerspectiveChildrenOverflow(const nsIFrame* aStartFrame);
 
+  /**
+   * Returns whether this frame is the anchor of some ancestor scroll frame. As
+   * this frame is moved, the scroll frame will apply adjustments to keep this
+   * scroll frame in the same relative position.
+   *
+   * aOutContainer will optionally be set to the scroll anchor container for
+   * this frame if this frame is an anchor.
+   */
+  bool IsScrollAnchor(
+      mozilla::layout::ScrollAnchorContainer** aOutContainer = nullptr);
+
+  /**
+   * Returns whether this frame is the anchor of some ancestor scroll frame, or
+   * has a descendant which is the scroll anchor.
+   */
+  bool IsInScrollAnchorChain() const;
+  void SetInScrollAnchorChain(bool aInChain);
+
   /**
    * Returns the number of ancestors between this and the root of our frame tree
    */
@@ -4293,9 +4316,12 @@ class nsIFrame : public nsQueryFrame {
    */
   bool mAllDescendantsAreInvisible : 1;
 
- protected:
-  // There is a 1-bit gap left here.
+  /**
+   * True if we are or contain the scroll anchor for a scrollable frame.
+   */
+  bool mInScrollAnchorChain : 1;
 
+ protected:
   // Helpers
   /**
    * Can we stop inside this frame when we're skipping non-rendered whitespace?