3 files changed, 39 insertions, 6 deletions

diff --git a/layout/base/nsIPresShell.h b/layout/base/nsIPresShell.h
index cf19867ef0..d368402e4f 100644
--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -486,6 +486,12 @@ public:
   virtual nsCanvasFrame* GetCanvasFrame() const = 0;
 
   /**
+  * Returns whether the scollable frame need to have a position adjustment
+  * based on scroll-anchoring.
+  */
+  virtual void ScrollableFrameNeedsAnchorAdjustment(nsIScrollableFrame* aFrame) = 0;
+
+  /**
    * Tell the pres shell that a frame needs to be marked dirty and needs
    * Reflow.  It's OK if this is an ancestor of the frame needing reflow as
    * long as the ancestor chain between them doesn't cross a reflow root.
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index 23876cc112..0fc84d736c 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -1225,6 +1225,7 @@ PresShell::Destroy()
   }
 
   mFramesToDirty.Clear();
+  mScrollAnchorAdjustments.Clear();
 
   if (mViewManager) {
     // Clear the view manager's weak pointer back to |this| in case it
@@ -2049,6 +2050,11 @@ PresShell::NotifyDestroyingFrame(nsIFrame* aFrame)
     }
 
     mFramesToDirty.RemoveEntry(aFrame);
+
+    nsIScrollableFrame* scrollableFrame = do_QueryFrame(aFrame);
+    if (scrollableFrame) {
+      mScrollAnchorAdjustments.RemoveEntry(scrollableFrame);
+    }
   } else {
     // We must delete this property in situ so that its destructor removes the
     // frame from FrameLayerBuilder::DisplayItemData::mFrameList -- otherwise
@@ -2584,6 +2590,12 @@ PresShell::VerifyHasDirtyRootAncestor(nsIFrame* aFrame)
 #endif
 
 void
+PresShell::ScrollableFrameNeedsAnchorAdjustment(nsIScrollableFrame* aFrame)
+{
+  mScrollAnchorAdjustments.PutEntry(aFrame);
+}
+
+void
 PresShell::FrameNeedsReflow(nsIFrame *aFrame, IntrinsicDirty aIntrinsicDirty,
                             nsFrameState aBitToAdd,
                             ReflowRootHandling aRootHandling)
@@ -4106,12 +4118,21 @@ PresShell::FlushPendingNotifications(mozilla::ChangesToFlush aFlush)
       didLayoutFlush = true;
       mFrameConstructor->RecalcQuotesAndCounters();
       viewManager->FlushDelayedResize(true);
-      if (ProcessReflowCommands(flushType < Flush_Layout) && mContentToScrollTo) {
-        // We didn't get interrupted.  Go ahead and scroll to our content
-        DoScrollContentIntoView();
+      if (ProcessReflowCommands(flushType < Flush_Layout)) {
+        // Apply scroll offset updates for scroll anchors.
+        for (auto iter = mScrollAnchorAdjustments.Iter(); !iter.Done(); iter.Next()) {
+          nsIScrollableFrame* frame = iter.Get()->GetKey();
+          frame->ApplyScrollAnchorOffsetAdjustment();
+        }
+        mScrollAnchorAdjustments.Clear();
+
         if (mContentToScrollTo) {
-          mContentToScrollTo->DeleteProperty(nsGkAtoms::scrolling);
-          mContentToScrollTo = nullptr;
+          // We didn't get interrupted; go ahead and scroll to our content.
+          DoScrollContentIntoView();
+          if (mContentToScrollTo) {
+            mContentToScrollTo->DeleteProperty(nsGkAtoms::scrolling);
+            mContentToScrollTo = nullptr;
+          }
         }
       }
     }
@@ -10833,6 +10854,7 @@ PresShell::AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf,
   }
   *aPresShellSize += mApproximatelyVisibleFrames.ShallowSizeOfExcludingThis(aMallocSizeOf);
   *aPresShellSize += mFramesToDirty.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  *aPresShellSize += mScrollAnchorAdjustments.ShallowSizeOfExcludingThis(aMallocSizeOf);
   *aPresShellSize += aArenaObjectsSize->mOther;
 
   if (nsStyleSet* styleSet = StyleSet()->GetAsGecko()) {
diff --git a/layout/base/nsPresShell.h b/layout/base/nsPresShell.h
index fbbcfc7ecd..de638f4bf6 100644
--- a/layout/base/nsPresShell.h
+++ b/layout/base/nsPresShell.h
@@ -129,6 +129,8 @@ public:
   virtual nsIPageSequenceFrame* GetPageSequenceFrame() const override;
   virtual nsCanvasFrame* GetCanvasFrame() const override;
 
+  virtual void ScrollableFrameNeedsAnchorAdjustment(nsIScrollableFrame* aFrame) override;
+  
   virtual void FrameNeedsReflow(nsIFrame *aFrame, IntrinsicDirty aIntrinsicDirty,
                                 nsFrameState aBitToAdd,
                                 ReflowRootHandling aRootHandling =
@@ -427,9 +429,10 @@ public:
 
   void SetNextPaintCompressed() { mNextPaintCompressed = true; }
 
-protected:
   virtual ~PresShell();
 
+protected:
+
   void HandlePostedReflowCallbacks(bool aInterruptible);
   void CancelPostedReflowCallbacks();
 
@@ -852,6 +855,8 @@ protected:
   // Set of frames that we should mark with NS_FRAME_HAS_DIRTY_CHILDREN after
   // we finish reflowing mCurrentReflowRoot.
   nsTHashtable<nsPtrHashKey<nsIFrame> > mFramesToDirty;
+  
+  nsTHashtable<nsPtrHashKey<nsIScrollableFrame> > mScrollAnchorAdjustments;
 
   // Reflow roots that need to be reflowed.
   nsTArray<nsIFrame*>       mDirtyRoots;