Don't invalidate width for children unless scrollbars have changed.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 157 matching lines @@
 
     // If a PreventRelayoutScope object is alive, updateAfterLayout() will not
     // re-run box layout as a result of adding or removing scrollbars.
     class PreventRelayoutScope {
         STACK_ALLOCATED();
     public:
         PreventRelayoutScope(SubtreeLayoutScope&);
         ~PreventRelayoutScope();
 
         static bool relayoutIsPrevented() { return s_count; }
-        static void setNeedsLayout(LayoutObject&);
+        static void setBoxNeedsLayout(PaintLayerScrollableArea&, bool hadHorizontalScrollbar, bool hadVerticalScrollbar);

[skobes, 2016/06/23 18:19:18]

This API is super confusing, if the box needs layout why does it matter whether
we had scrollbars?  Should it be named notifyScrollbarsChanged or similar?

[szager1, 2016/06/23 19:05:31]

There are two things going on here:

- Does the box need relayout?
- If so, should the box force relayout of its children?

setBoxNeedsLayout affirms the first condition, i.e., the box definitely needs
relayout.  The second question can't be answered until we know the final state
of scrollbars, which won't be until s_count drops to zero.

I added to the class comment, hopefully that helps.  This is indeed pretty
gross, although I do think these classes do a good job of hiding the grossness
from LayoutFlexibleBox, which has enough of its own grossness already.

I share your optimism about layout ng.

         static bool relayoutNeeded() { return s_count == 0 && s_relayoutNeeded; }
         static void resetRelayoutNeeded();
 
     private:
         static int s_count;
         static SubtreeLayoutScope* s_layoutScope;
         static bool s_relayoutNeeded;
-        static WTF::Vector<LayoutObject*>* s_needsRelayout;
+        static PersistentHeapVector<Member<PaintLayerScrollableArea>>* s_needsRelayout;
     };
 
     // If a FreezeScrollbarScope object is alive, updateAfterLayout() will not
     // recompute the existence of overflow:auto scrollbars.
     class FreezeScrollbarsScope {
         STACK_ALLOCATED();
     public:
         FreezeScrollbarsScope() { s_count++; }
         ~FreezeScrollbarsScope() { s_count--; }
 
@@ skipping 206 matching lines @@
     Widget* getWidget() override;
     ScrollAnchor& scrollAnchor() { return m_scrollAnchor; }
     bool isPaintLayerScrollableArea() const override { return true; }
 
     bool shouldRebuildHorizontalScrollbarLayer() const { return m_rebuildHorizontalScrollbarLayer; }
     bool shouldRebuildVerticalScrollbarLayer() const { return m_rebuildVerticalScrollbarLayer; }
     void resetRebuildScrollbarLayerFlags();
 
     bool needsScrollPositionClamp() const { return m_needsScrollPositionClamp; }
     void setNeedsScrollPositionClamp(bool val) { m_needsScrollPositionClamp = val; }
+    bool needsRelayout() const { return m_needsRelayout; }
+    void setNeedsRelayout(bool val) { m_needsRelayout = val; }
+    bool hadHorizontalScrollbarBeforeRelayout() const { return m_hadHorizontalScrollbarBeforeRelayout; }

[skobes, 2016/06/23 18:19:18]

Do these need to be public?  Add comments please.

[szager1, 2016/06/23 19:05:31]

I could use a friend declaration to give PreventRelayoutScope access to private
members, but that's not a common idiom in blink code.

Comments added.

+    void setHadHorizontalScrollbarBeforeRelayout(bool val) { m_hadHorizontalScrollbarBeforeRelayout = val; }
+    bool hadVerticalScrollbarBeforeRelayout() const { return m_hadVerticalScrollbarBeforeRelayout; }
+    void setHadVerticalScrollbarBeforeRelayout(bool val) { m_hadVerticalScrollbarBeforeRelayout = val; }
 
     StickyConstraintsMap& stickyConstraintsMap() { return ensureRareData().m_stickyConstraintsMap; }
     void invalidateAllStickyConstraints();
     void invalidateStickyConstraintsFor(PaintLayer*, bool needsCompositingUpdate = true);
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit PaintLayerScrollableArea(PaintLayer&);
 
@@ skipping 38 matching lines @@
         if (!m_rareData)
             m_rareData = wrapUnique(new PaintLayerScrollableAreaRareData());
         return *m_rareData.get();
     }
 
     // PaintInvalidationCapableScrollableArea
     LayoutBox& boxForScrollControlPaintInvalidation() const { return box(); }
 
     PaintLayer& m_layer;
 
+    PaintLayer* m_nextTopmostScrollChild;
+    PaintLayer* m_topmostScrollChild;
+
     // Keeps track of whether the layer is currently resizing, so events can cause resizing to start and stop.
     unsigned m_inResizeMode : 1;
     unsigned m_scrollsOverflow : 1;
 
     unsigned m_inOverflowRelayout : 1;
 
-    PaintLayer* m_nextTopmostScrollChild;
-    PaintLayer* m_topmostScrollChild;
-
     // FIXME: once cc can handle composited scrolling with clip paths, we will
     // no longer need this bit.
     unsigned m_needsCompositedScrolling : 1;
 
     // Set to indicate that a scrollbar layer, if present, needs to be rebuilt
     // in the next compositing update because the underlying blink::Scrollbar
     // instance has been reconstructed.
     unsigned m_rebuildHorizontalScrollbarLayer : 1;
     unsigned m_rebuildVerticalScrollbarLayer : 1;
 
     unsigned m_needsScrollPositionClamp : 1;
+    unsigned m_needsRelayout : 1;

[skobes, 2016/06/23 18:19:18]

Please add comments explaining what this means.  How is it different from
box().needsLayout()?

[szager1, 2016/06/23 19:05:31]

I added comments to the getters/setters.

+    unsigned m_hadHorizontalScrollbarBeforeRelayout : 1;
+    unsigned m_hadVerticalScrollbarBeforeRelayout : 1;
 
     // The width/height of our scrolled area.
     // This is OverflowModel's layout overflow translated to physical
     // coordinates. See OverflowModel for the different overflow and
     // LayoutBoxModelObject for the coordinate systems.
     LayoutRect m_overflowRect;
 
     // ScrollbarManager holds the Scrollbar instances.
     ScrollbarManager m_scrollbarManager;
 
@@ skipping 17 matching lines @@
 #endif
 };
 
 DEFINE_TYPE_CASTS(PaintLayerScrollableArea, ScrollableArea, scrollableArea,
     scrollableArea->isPaintLayerScrollableArea(),
     scrollableArea.isPaintLayerScrollableArea());
 
 } // namespace blink
 
 #endif // LayerScrollableArea_h