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 150 matching lines @@
         Member<Scrollbar> m_vBar;
 
         unsigned m_hBarIsAttached: 1;
         unsigned m_vBarIsAttached: 1;
     };
 
 public:
 
     // If a PreventRelayoutScope object is alive, updateAfterLayout() will not
     // re-run box layout as a result of adding or removing scrollbars.
+    // Instead, it will mark the PLSA as needing relayout of its box.
+    // When the last PreventRelayoutScope object is popped off the stack,
+    // box().setNeedsLayout(), and box().scrollbarsChanged() for LayoutBlock's,
+    // will be called as appropriate for all marked PLSA's.

[skobes, 2016/06/23 19:08:22]

thanks, this helps a lot

     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);
         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 204 matching lines @@
     IntRect rectForVerticalScrollbar(const IntRect& borderBoxRect) const;
 
     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();
 
+    // Did DelayScrollPositionClampScope prevent us from running clampScrollPositionsAfterLayout()
+    // in updateAfterLayout()?
     bool needsScrollPositionClamp() const { return m_needsScrollPositionClamp; }
     void setNeedsScrollPositionClamp(bool val) { m_needsScrollPositionClamp = val; }
 
+    // Did PreventRelayoutScope prevent us from running re-layout due to adding/subtracting
+    // scrollbars in updateAfterLayout()?
+    bool needsRelayout() const { return m_needsRelayout; }
+    void setNeedsRelayout(bool val) { m_needsRelayout = val; }
+
+    // Were we laid out with a horizontal scrollbar at the time we were marked as
+    // needing relayout by PreventRelayoutScope?
+    bool hadHorizontalScrollbarBeforeRelayout() const { return m_hadHorizontalScrollbarBeforeRelayout; }
+    void setHadHorizontalScrollbarBeforeRelayout(bool val) { m_hadHorizontalScrollbarBeforeRelayout = val; }
+
+    // Were we laid out with a vertical scrollbar at the time we were marked as
+    // needing relayout by PreventRelayoutScope?
+    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&);
 
     bool hasHorizontalOverflow() const;
@@ skipping 37 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;
+    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