Bug 1305957 part 8 - Suppress scroll offset adjustment when some layout affecting properties are changed on scroll anchor or its ancestors. r=hiro

This commit implements the first half of the heuristics to detect style changes
that could lead to feedback loops with scroll anchoring. [1]

When these style changes are made, a suppression flag is added to the anchor
container to ignore any adjustments that would be made at the next layout flush
and to invalidate the anchor at that time.

[1] https://drafts.csswg.org/css-scroll-anchoring/#suppression-triggers

Differential Revision: https://phabricator.services.mozilla.com/D13271

--HG--
extra : rebase_source : db0bf373e51325b7ef537c4c245ede4b4219d0d7
extra : source : d9445a5f3458b560fbdb7aee5faabb5a7a9327f3

layout/generic/ScrollAnchorContainer.cpp

@@ -21,7 +21,8 @@ ScrollAnchorContainer::ScrollAnchorContainer(ScrollFrameHelper* aScrollFrame)
       mAnchorNode(nullptr),
       mLastAnchorPos(0, 0),
       mAnchorNodeIsDirty(true),
-      mApplyingAnchorAdjustment(false) {}
+      mApplyingAnchorAdjustment(false),
+      mSuppressAnchorAdjustment(false) {}
 
 ScrollAnchorContainer::~ScrollAnchorContainer() {}
 

layout/generic/ScrollAnchorContainer.h

@@ -71,6 +71,12 @@ class ScrollAnchorContainer final {
    */
   void ApplyAdjustments();
 
+  /**
+   * Notify the scroll anchor container that it should suppress any scroll
+   * adjustment that may happen after the next layout flush.
+   */
+  void SuppressAdjustments();
+
   /**
    * Notify this scroll anchor container that its anchor node should be
    * invalidated and recomputed at the next available opportunity.
@@ -135,6 +141,8 @@ class ScrollAnchorContainer final {
   bool mAnchorNodeIsDirty : 1;
   // True if we are applying a scroll anchor adjustment
   bool mApplyingAnchorAdjustment : 1;
+  // True if we should suppress anchor adjustments
+  bool mSuppressAnchorAdjustment : 1;
 };
 
 }  // namespace layout

layout/generic/nsFrame.cpp

@@ -1053,6 +1053,11 @@ void nsIFrame::MarkNeedsDisplayItemRebuild() {
   AddAndRemoveImageAssociations(this, oldLayers, newLayers);
 
   if (aOldComputedStyle) {
+    // Detect style changes that should trigger a scroll anchor adjustment
+    // suppression.
+    // https://drafts.csswg.org/css-scroll-anchoring/#suppression-triggers
+    bool needAnchorSuppression = false;
+
     // If we detect a change on margin, padding or border, we store the old
     // values on the frame itself between now and reflow, so if someone
     // calls GetUsed(Margin|Border|Padding)() before the next reflow, we
@@ -1062,17 +1067,21 @@ void nsIFrame::MarkNeedsDisplayItemRebuild() {
     nsMargin newValue(0, 0, 0, 0);
     const nsStyleMargin* oldMargin = aOldComputedStyle->PeekStyleMargin();
     if (oldMargin && oldMargin->GetMargin(oldValue)) {
-      if ((!StyleMargin()->GetMargin(newValue) || oldValue != newValue) &&
-          !HasProperty(UsedMarginProperty())) {
-        AddProperty(UsedMarginProperty(), new nsMargin(oldValue));
+      if (!StyleMargin()->GetMargin(newValue) || oldValue != newValue) {
+        if (!HasProperty(UsedMarginProperty())) {
+          AddProperty(UsedMarginProperty(), new nsMargin(oldValue));
+        }
+        needAnchorSuppression = true;
       }
     }
 
     const nsStylePadding* oldPadding = aOldComputedStyle->PeekStylePadding();
     if (oldPadding && oldPadding->GetPadding(oldValue)) {
-      if ((!StylePadding()->GetPadding(newValue) || oldValue != newValue) &&
-          !HasProperty(UsedPaddingProperty())) {
-        AddProperty(UsedPaddingProperty(), new nsMargin(oldValue));
+      if (!StylePadding()->GetPadding(newValue) || oldValue != newValue) {
+        if (!HasProperty(UsedPaddingProperty())) {
+          AddProperty(UsedPaddingProperty(), new nsMargin(oldValue));
+        }
+        needAnchorSuppression = true;
       }
     }
 
@@ -1084,6 +1093,31 @@ void nsIFrame::MarkNeedsDisplayItemRebuild() {
         AddProperty(UsedBorderProperty(), new nsMargin(oldValue));
       }
     }
+
+    if (mInScrollAnchorChain) {
+      const nsStylePosition* oldPosition =
+          aOldComputedStyle->PeekStylePosition();
+      if (oldPosition &&
+          (oldPosition->mOffset != StylePosition()->mOffset ||
+           oldPosition->mWidth != StylePosition()->mWidth ||
+           oldPosition->mMinWidth != StylePosition()->mMinWidth ||
+           oldPosition->mMaxWidth != StylePosition()->mMaxWidth ||
+           oldPosition->mHeight != StylePosition()->mHeight ||
+           oldPosition->mMinHeight != StylePosition()->mMinHeight ||
+           oldPosition->mMaxHeight != StylePosition()->mMaxHeight)) {
+        needAnchorSuppression = true;
+      }
+
+      const nsStyleDisplay* oldDisp = aOldComputedStyle->PeekStyleDisplay();
+      if (oldDisp && (oldDisp->mPosition != StyleDisplay()->mPosition ||
+                      oldDisp->TransformChanged(*StyleDisplay()))) {
+        needAnchorSuppression = true;
+      }
+    }
+
+    if (mInScrollAnchorChain && needAnchorSuppression) {
+      ScrollAnchorContainer::FindFor(this)->SuppressAdjustments();
+    }
   }
 
   ImageLoader* imageLoader = PresContext()->Document()->StyleImageLoader();

layout/style/nsStyleStruct.cpp

@@ -3147,6 +3147,12 @@ void nsStyleDisplay::FinishStyle(nsPresContext* aPresContext,
   GenerateCombinedIndividualTransform();
 }
 
+static inline bool TransformListChanged(
+    const RefPtr<nsCSSValueSharedList>& aList,
+    const RefPtr<nsCSSValueSharedList>& aNewList) {
+  return !aList != !aNewList || (aList && *aList != *aNewList);
+}
+
 static inline nsChangeHint CompareTransformValues(
     const RefPtr<nsCSSValueSharedList>& aList,
     const RefPtr<nsCSSValueSharedList>& aNewList) {
@@ -3425,6 +3431,11 @@ nsChangeHint nsStyleDisplay::CalcDifference(
   return hint;
 }
 
+bool nsStyleDisplay::TransformChanged(const nsStyleDisplay& aNewData) const {
+  return TransformListChanged(mSpecifiedTransform,
+                              aNewData.mSpecifiedTransform);
+}
+
 void nsStyleDisplay::GenerateCombinedIndividualTransform() {
   // FIXME(emilio): This should probably be called from somewhere like what we
   // do for image layers, instead of FinishStyle.

layout/style/nsStyleStruct.h

@@ -1881,6 +1881,8 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleDisplay {
 
   nsChangeHint CalcDifference(const nsStyleDisplay& aNewData) const;
 
+  bool TransformChanged(const nsStyleDisplay& aNewData) const;
+
   // We guarantee that if mBinding is non-null, so are mBinding->GetURI() and
   // mBinding->mOriginPrincipal.
   RefPtr<mozilla::css::URLValue> mBinding;