Tie scroll anchoring adjustments to frame lifecycle instead of layout.

third_party/WebKit/Source/core/layout/ScrollAnchor.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ScrollAnchor_h
 #define ScrollAnchor_h
 
 #include "core/CoreExport.h"
 #include "platform/geometry/LayoutPoint.h"
 #include "platform/heap/Handle.h"
@@ skipping 13 matching lines @@
   ~ScrollAnchor();
 
   // The scroller that is scrolled to componsate for layout movements. Note
   // that the scroller can only be initialized once.
   void setScroller(ScrollableArea*);
 
   // Returns true if the underlying scroller is set.
   bool hasScroller() const { return m_scroller; }
 
   // The LayoutObject we are currently anchored to. Lazily computed during
-  // save() and cached until the next call to clear().
+  // notifyBeforeLayout() and cached until the next call to clear().
   LayoutObject* anchorObject() const { return m_anchorObject; }
 
-  // Indicates that the next save() should compute a new anchor for the
-  // containing scroller and all ancestor scrollers.
+  // Indicates that this ScrollAnchor, and all ancestor ScrollAnchors, should
+  // compute new anchor nodes on their next notifyBeforeLayout().
   void clear();
 
   // Records the anchor's location in relation to the scroller. Should be
   // called when the scroller is about to be laid out.
-  void save();
+  void notifyBeforeLayout();
 
-  // Scrolls to compensate for any change in the anchor's relative location
-  // since the most recent call to save(). Should be called immediately after
-  // the scroller has been laid out.
-  void restore();
+  // Scrolls to compensate for any change in the anchor's relative location.
+  // Should be called at the end of the animation frame.
+  void adjust();
 
   enum class Corner {
     TopLeft = 0,
     TopRight,
   };
   // Which corner of the anchor object we are currently anchored to.
   // Only meaningful if anchorObject() is non-null.
   Corner corner() const { return m_corner; }
 
   // Checks if we hold any references to the specified object.
   bool refersTo(const LayoutObject*) const;
 
   // Notifies us that an object will be removed from the layout tree.
   void notifyRemoved(LayoutObject*);
 
   DEFINE_INLINE_TRACE() { visitor->trace(m_scroller); }
 
  private:
-  // Indicates that the next save() should compute a new anchor for the
-  // containing scroller.
+  // Indicates that this ScrollAnchor should compute a new anchor node on the
+  // next call to notifyBeforeLayout().
   void clearSelf(bool unconditionally = false);
 
   void findAnchor();
   bool computeScrollAnchorDisablingStyleChanged();
 
   enum WalkStatus { Skip = 0, Constrain, Continue, Return };
   struct ExamineResult {
     ExamineResult(WalkStatus s)
         : status(s), viable(false), corner(Corner::TopLeft) {}
 
     ExamineResult(WalkStatus s, Corner c)
         : status(s), viable(true), corner(c) {}
 
     WalkStatus status;
     bool viable;
     Corner corner;
   };
   ExamineResult examine(const LayoutObject*) const;
 
   IntSize computeAdjustment() const;
 
-  // The scroller that owns and is adjusted by this ScrollAnchor.
+  // The scroller to be adjusted by this ScrollAnchor. This is also the scroller
+  // that owns us, unless it is the RootFrameViewport in which case we are owned
+  // by the main frame's FrameView.

[ymalik, 2016/10/13 13:05:40]

s/owned by the main frame's FrameView/owned by RootFrameViewport's
m_layoutViewport, which in the general case is the main frame's FrameView?

[bokan, 2016/10/13 14:23:57]

Right, if we swapped the layout viewport using document.rootScroller, we might
be owned by a different FrameView or PLSA but m_scroller will still be
RootFrameViewport.

[skobes, 2016/10/13 17:21:37]

Done.

[skobes, 2016/10/13 17:21:37]

Acknowledged.

   Member<ScrollableArea> m_scroller;
 
   // The LayoutObject we should anchor to.
   LayoutObject* m_anchorObject;
 
   // Which corner of m_anchorObject's bounding box to anchor to.
   Corner m_corner;
 
-  // Location of m_layoutObject relative to scroller at time of save().
+  // Location of m_layoutObject relative to scroller at time of
+  // notifyBeforeLayout().
   LayoutPoint m_savedRelativeOffset;
 
   // We suppress scroll anchoring after a style change on the anchor node or
   // one of its ancestors, if that change might have caused the node to move.
   // This bit tracks whether we have had a scroll-anchor-disabling style
-  // change since the last layout.  It is recomputed in save(), and used to
-  // suppress the adjustment in restore().  More at http://bit.ly/sanaclap.
+  // change since the last layout.  It is recomputed in notifyBeforeLayout(),
+  // and used to suppress adjustment in adjust().  See http://bit.ly/sanaclap.
   bool m_scrollAnchorDisablingStyleChanged;
 
-  // True iff save has been called, and restore has not been called since
-  // the call to save.  In this state, additional calls to save are ignored,
-  // to make things easier for multi-pass layout modes such as flexbox.
-  // TODO(skobes): explore anchoring at frame boundaries instead of layouts,
-  // which would allow this field to be removed.
-  bool m_saved;
+  // True iff an adjustment check has been queued with the FrameView but not yet
+  // performed.
+  bool m_queued;
 };
 
 }  // namespace blink
 
 #endif  // ScrollAnchor_h