Bug 1871760 - Revert bug 1856088. r=botond

Two test cases attached in bug 1856088 now work with the previous commit and
work without `MergeableAbsolute`.

Differential Revision: https://phabricator.services.mozilla.com/D199841

gfx/layers/FrameMetrics.cpp

@@ -147,12 +147,10 @@ CSSSize FrameMetrics::CalculateCompositedSizeInCssPixels(
   return aCompositionBounds.Size() / aZoom;
 }
 
-std::pair<bool, CSSPoint> FrameMetrics::ApplyAbsoluteScrollUpdateFrom(
-    const ScrollPositionUpdate& aUpdate) {
-  CSSPoint oldVisualOffset = GetVisualScrollOffset();
+bool FrameMetrics::ApplyScrollUpdateFrom(const ScrollPositionUpdate& aUpdate) {
   // In applying a main-thread scroll update, try to preserve the relative
   // offset between the visual and layout viewports.
-  CSSPoint relativeOffset = oldVisualOffset - GetLayoutScrollOffset();
+  CSSPoint relativeOffset = GetVisualScrollOffset() - GetLayoutScrollOffset();
   MOZ_ASSERT(IsRootContent() || relativeOffset == CSSPoint());
   // We need to set the two offsets together, otherwise a subsequent
   // RecalculateLayoutViewportOffset() could see divergent layout and
@@ -160,7 +158,7 @@ std::pair<bool, CSSPoint> FrameMetrics::ApplyAbsoluteScrollUpdateFrom(
   bool offsetChanged = SetLayoutScrollOffset(aUpdate.GetDestination());
   offsetChanged |=
       ClampAndSetVisualScrollOffset(aUpdate.GetDestination() + relativeOffset);
-  return {offsetChanged, GetVisualScrollOffset() - oldVisualOffset};
+  return offsetChanged;
 }
 
 CSSPoint FrameMetrics::ApplyRelativeScrollUpdateFrom(

gfx/layers/FrameMetrics.h

@@ -243,11 +243,9 @@ struct FrameMetrics {
   }
 
   /*
-   * Returns true if the layout scroll offset or visual scroll offset changed
-   * and returns the visual scroll offset change delta.
+   * Returns true if the layout scroll offset or visual scroll offset changed.
    */
-  std::pair<bool, CSSPoint> ApplyAbsoluteScrollUpdateFrom(
-      const ScrollPositionUpdate& aUpdate);
+  bool ApplyScrollUpdateFrom(const ScrollPositionUpdate& aUpdate);
 
   /**
    * Applies the relative scroll offset update contained in aOther to the

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -5732,20 +5732,11 @@ void AsyncPanZoomController::NotifyLayersUpdated(
       relativeDelta =
           Some(Metrics().ApplyPureRelativeScrollUpdateFrom(scrollUpdate));
       Metrics().RecalculateLayoutViewportOffset();
-    } else if (scrollUpdate.GetType() == ScrollUpdateType::MergeableAbsolute) {
-      APZC_LOG("%p mergeable updating scroll offset from %s to %s\n", this,
-               ToString(Metrics().GetVisualScrollOffset()).c_str(),
-               ToString(scrollUpdate.GetDestination()).c_str());
-      relativeDelta =
-          Some(Metrics().ApplyAbsoluteScrollUpdateFrom(scrollUpdate).second);
-      Metrics().RecalculateLayoutViewportOffset();
-      scrollOffsetUpdated = true;
     } else {
       APZC_LOG("%p updating scroll offset from %s to %s\n", this,
                ToString(Metrics().GetVisualScrollOffset()).c_str(),
                ToString(scrollUpdate.GetDestination()).c_str());
-      auto [offsetChanged, _] =
-          Metrics().ApplyAbsoluteScrollUpdateFrom(scrollUpdate);
+      bool offsetChanged = Metrics().ApplyScrollUpdateFrom(scrollUpdate);
       Metrics().RecalculateLayoutViewportOffset();
 
       if (offsetChanged || scrollUpdate.GetMode() != ScrollMode::Instant ||

gfx/layers/ipc/LayersMessageUtils.h

@@ -520,7 +520,7 @@ template <>
 struct ParamTraits<mozilla::ScrollUpdateType>
     : public ContiguousEnumSerializerInclusive<
           mozilla::ScrollUpdateType, mozilla::ScrollUpdateType::Absolute,
-          mozilla::ScrollUpdateType::MergeableAbsolute> {};
+          mozilla::ScrollUpdateType::PureRelative> {};
 
 template <>
 struct ParamTraits<mozilla::ScrollMode>

layout/generic/ScrollAnchorContainer.cpp

@@ -539,11 +539,8 @@ void ScrollAnchorContainer::ApplyAdjustments() {
   MOZ_RELEASE_ASSERT(!mApplyingAnchorAdjustment);
   // We should use AutoRestore here, but that doesn't work with bitfields
   mApplyingAnchorAdjustment = true;
-  Frame()->ScrollToInternal(
-      Frame()->GetScrollPosition() + physicalAdjustment, ScrollMode::Instant,
-      StaticPrefs::layout_css_scroll_anchoring_absolute_update()
-          ? ScrollOrigin::AnchorAdjustment
-          : ScrollOrigin::Relative);
+  Frame()->ScrollToInternal(Frame()->GetScrollPosition() + physicalAdjustment,
+                            ScrollMode::Instant, ScrollOrigin::Relative);
   mApplyingAnchorAdjustment = false;
 
   if (Frame()->mIsRoot) {

layout/generic/ScrollOrigin.h

@@ -33,8 +33,6 @@ enum class ScrollOrigin : uint8_t {
   // The scroll came from an attempt by the main thread to re-clamp the scroll
   // position after a reflow.
   Clamp,
-  // The scroll came from a scroll adjustment triggered by scroll anchoring.
-  AnchorAdjustment,
 
   // The following scroll origins also are associated with prefs of the form
   //   general.smoothScroll.<origin>(.*)

layout/generic/ScrollPositionUpdate.cpp

@@ -93,20 +93,6 @@ ScrollPositionUpdate ScrollPositionUpdate::NewPureRelativeScroll(
   return ret;
 }
 
-/*static*/
-ScrollPositionUpdate ScrollPositionUpdate::NewMergeableScroll(
-    ScrollOrigin aOrigin, nsPoint aDestination) {
-  MOZ_ASSERT(aOrigin == ScrollOrigin::AnchorAdjustment);
-
-  ScrollPositionUpdate ret;
-  ret.mScrollGeneration = sGenerationCounter.NewMainThreadGeneration();
-  ret.mType = ScrollUpdateType::MergeableAbsolute;
-  ret.mScrollMode = ScrollMode::Instant;
-  ret.mScrollOrigin = aOrigin;
-  ret.mDestination = CSSPoint::FromAppUnits(aDestination);
-  return ret;
-}
-
 bool ScrollPositionUpdate::operator==(
     const ScrollPositionUpdate& aOther) const {
   // instances are immutable, and all the fields are set when the generation
@@ -126,7 +112,6 @@ ScrollOrigin ScrollPositionUpdate::GetOrigin() const { return mScrollOrigin; }
 
 CSSPoint ScrollPositionUpdate::GetDestination() const {
   MOZ_ASSERT(mType == ScrollUpdateType::Absolute ||
-             mType == ScrollUpdateType::MergeableAbsolute ||
              mType == ScrollUpdateType::Relative);
   return mDestination;
 }

layout/generic/ScrollPositionUpdate.h

@@ -34,9 +34,6 @@ enum class ScrollUpdateType {
   // The delta should be applied to whatever the current scroll position is
   // on the receiver side.
   PureRelative,
-  // Similar to |Absolute|, but even if there's an active async scroll animation
-  // the position update will NOT cancel the async scroll animation.
-  MergeableAbsolute,
 };
 
 enum class ScrollTriggeredByScript : bool { No, Yes };
@@ -86,9 +83,6 @@ class ScrollPositionUpdate {
                                                     ScrollMode aMode,
                                                     const nsPoint& aDelta);
 
-  static ScrollPositionUpdate NewMergeableScroll(ScrollOrigin aOrigin,
-                                                 nsPoint aDestination);
-
   bool operator==(const ScrollPositionUpdate& aOther) const;
 
   MainThreadScrollGeneration GetGeneration() const;

layout/generic/nsGfxScrollFrame.cpp

@@ -2245,12 +2245,7 @@ void nsHTMLScrollFrame::AsyncScroll::InitSmoothScroll(
     case ScrollOrigin::Apz:
       // Likewise we should never get APZ-triggered scrolls here, and if that
       // changes something is likely broken somewhere.
-      MOZ_ASSERT_UNREACHABLE(
-          "APZ scroll position updates should never be smooth");
-      break;
-    case ScrollOrigin::AnchorAdjustment:
-      MOZ_ASSERT_UNREACHABLE(
-          "scroll anchor adjustments should never be smooth");
+      MOZ_ASSERT(false);
       break;
     default:
       break;
@@ -3026,7 +3021,6 @@ void nsHTMLScrollFrame::ScrollToImpl(
       (mLastScrollOrigin != ScrollOrigin::None &&
        mLastScrollOrigin != ScrollOrigin::NotSpecified &&
        mLastScrollOrigin != ScrollOrigin::Relative &&
-       mLastScrollOrigin != ScrollOrigin::AnchorAdjustment &&
        mLastScrollOrigin != ScrollOrigin::Apz)) {
     aOrigin = ScrollOrigin::Other;
   }
@@ -3170,9 +3164,6 @@ void nsHTMLScrollFrame::ScrollToImpl(
     AppendScrollUpdate(
         ScrollPositionUpdate::NewRelativeScroll(mApzScrollPos, pt));
     mApzScrollPos = pt;
-  } else if (aOrigin == ScrollOrigin::AnchorAdjustment) {
-    AppendScrollUpdate(ScrollPositionUpdate::NewMergeableScroll(aOrigin, pt));
-    mApzScrollPos = pt;
   } else if (aOrigin != ScrollOrigin::Apz) {
     AppendScrollUpdate(ScrollPositionUpdate::NewScroll(mLastScrollOrigin, pt));
   }

modules/libpref/init/StaticPrefList.yaml

@@ -9223,12 +9223,6 @@
   value: true
   mirror: always
 
-# Handle scroll-anchoring adjustments as absolute scroll position updates.
-- name: layout.css.scroll-anchoring.absolute-update
-  type: bool
-  value: true
-  mirror: always
-
 # Are shared memory User Agent style sheets enabled?
 - name: layout.css.shared-memory-ua-sheets.enabled
   type: bool