Avoid scrollbar construction/destruction thrashing during flex layout.

third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.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 49 matching lines @@
 class PlatformEvent;
 class LayoutBox;
 class DeprecatedPaintLayer;
 class LayoutScrollbarPart;
 
 class CORE_EXPORT DeprecatedPaintLayerScrollableArea final : public NoBaseWillBeGarbageCollectedFinalized<DeprecatedPaintLayerScrollableArea>, public ScrollableArea {
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED(DeprecatedPaintLayerScrollableArea);
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(DeprecatedPaintLayerScrollableArea);
     friend class Internals;
 
+private:
+    class ScrollbarManager {
+        // Helper class to manage the life cycle of Scrollbar objects.  Some layout containers
+        // (e.g., flexbox, table) run multi-pass layout on their children, applying different
+        // constraints.  If a child has overflow:auto, it may gain and lose scrollbars multiple
+        // times during multi-pass layout, causing pointless allocation/deallocation thrashing,
+        // and potentially leading to other problems (crbug.com/528940).
+
+        // ScrollbarManager allows a ScrollableArea to delay the destruction of a scrollbar that
+        // is no longer needed, until the end of multi-pass layout.  If the scrollbar is then
+        // re-added before multi-pass layout finishes, the previously "deleted" scrollbar will
+        // be restored, rather than constructing a new one.
+    public:
+        ScrollbarManager(DeprecatedPaintLayerScrollableArea&);
+
+        void dispose();
+
+        bool canDetachScrollbars() const { return m_canDetachScrollbars; }
+        void setCanDetachScrollbars(bool);

[skobes, 2015/09/26 01:57:06]

Add a comment here that documents what this api does (from the caller's
perspective).

[szager1, 2015/09/26 04:48:31]

Done.

+        Scrollbar* horizontalScrollbar() const { return m_hBarIsAttached ? m_hBar.get(): nullptr; }
+        Scrollbar* verticalScrollbar() const { return m_vBarIsAttached ? m_vBar.get() : nullptr; }
+        bool hasHorizontalScrollbar() const { return horizontalScrollbar(); }
+        bool hasVerticalScrollbar() const { return verticalScrollbar(); }
+
+        void setHasHorizontalScrollbar(bool hasScrollbar);
+        void setHasVerticalScrollbar(bool hasScrollbar);
+
+        DECLARE_TRACE();
+
+    private:
+        PassRefPtrWillBeRawPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
+        void destroyScrollbar(ScrollbarOrientation, bool invalidate = false);
+
+    private:
+        DeprecatedPaintLayerScrollableArea& m_scrollableArea;
+        RefPtrWillBeMember<Scrollbar> m_hBar;
+        RefPtrWillBeMember<Scrollbar> m_vBar;
+        unsigned m_canDetachScrollbars: 1;

[skobes, 2015/09/26 01:57:06]

Is there a reason to prefer unsigned:1 to bool?

[szager1, 2015/09/26 04:48:31]

No reason, just following convention.

[szager1, 2015/09/26 04:53:26]

Actually, after reading some StackOverflow, I retract my previous comment.  bool
member variables are always assigned 8 bits, because they must be
byte-addressable.  But integer types with a specified number of bits are not
byte-addressable, and they are packed efficiently.

+        unsigned m_hBarIsAttached: 1;
+        unsigned m_vBarIsAttached: 1;
+    };
+
 public:
     // FIXME: We should pass in the LayoutBox but this opens a window
     // for crashers during DeprecatedPaintLayer setup (see crbug.com/368062).
     static PassOwnPtrWillBeRawPtr<DeprecatedPaintLayerScrollableArea> create(DeprecatedPaintLayer& layer)
     {
         return adoptPtrWillBeNoop(new DeprecatedPaintLayerScrollableArea(layer));
     }
 
     ~DeprecatedPaintLayerScrollableArea() override;
     void dispose();
 
     bool hasHorizontalScrollbar() const { return horizontalScrollbar(); }
     bool hasVerticalScrollbar() const { return verticalScrollbar(); }
 
-    Scrollbar* horizontalScrollbar() const override { return m_hBar.get(); }
-    Scrollbar* verticalScrollbar() const override { return m_vBar.get(); }
+    Scrollbar* horizontalScrollbar() const override { return m_scrollbarManager.horizontalScrollbar(); }
+    Scrollbar* verticalScrollbar() const override { return m_scrollbarManager.verticalScrollbar(); }
 
     HostWindow* hostWindow() const override;
 
     GraphicsLayer* layerForScrolling() const override;
     GraphicsLayer* layerForHorizontalScrollbar() const override;
     GraphicsLayer* layerForVerticalScrollbar() const override;
     GraphicsLayer* layerForScrollCorner() const override;
     bool usesCompositedScrolling() const override;
     void invalidateScrollbarRect(Scrollbar*, const IntRect&) override;
     void invalidateScrollCornerRect(const IntRect&) override;
@@ skipping 59 matching lines @@
 
     void updateScrollDimensions(DoubleSize& scrollOffset, bool& autoHorizontalScrollBarChanged, bool& autoVerticalScrollBarChanged);
     void finalizeScrollDimensions(const DoubleSize& originalScrollOffset, bool autoHorizontalScrollBarChanged, bool autoVerticalScrollBarChanged);
 
     void updateAfterLayout();
     void updateAfterStyleChange(const ComputedStyle*);
     void updateAfterOverflowRecalc();
 
     bool updateAfterCompositingChange() override;
 
-    bool hasScrollbar() const { return m_hBar || m_vBar; }
+    bool hasScrollbar() const { return hasHorizontalScrollbar() || hasVerticalScrollbar(); }
 
     LayoutScrollbarPart* scrollCorner() const { return m_scrollCorner; }
 
     void resize(const PlatformEvent&, const LayoutSize&);
     IntSize offsetFromResizeCorner(const IntPoint& absolutePoint) const;
 
     bool inResizeMode() const { return m_inResizeMode; }
     void setInResizeMode(bool inResizeMode) { m_inResizeMode = inResizeMode; }
 
     IntRect touchResizerCornerRect(const IntRect& bounds) const
@@ skipping 71 matching lines @@
 
     void computeScrollDimensions();
 
     void setScrollOffset(const IntPoint&, ScrollType) override;
     void setScrollOffset(const DoublePoint&, ScrollType) override;
 
     LayoutUnit verticalScrollbarStart(int minX, int maxX) const;
     LayoutUnit horizontalScrollbarStart(int minX) const;
     IntSize scrollbarOffset(const Scrollbar*) const;
 
-    PassRefPtrWillBeRawPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
-    void destroyScrollbar(ScrollbarOrientation);
-
     void setHasHorizontalScrollbar(bool hasScrollbar);
     void setHasVerticalScrollbar(bool hasScrollbar);
 
     void updateScrollCornerStyle();
 
     // See comments on isPointInResizeControl.
     void updateResizerAreaSet();
     void updateResizerStyle();
 
 
@@ skipping 12 matching lines @@
     DeprecatedPaintLayer* m_nextTopmostScrollChild;
     DeprecatedPaintLayer* m_topmostScrollChild;
 
     // FIXME: once cc can handle composited scrolling with clip paths, we will
     // no longer need this bit.
     unsigned m_needsCompositedScrolling : 1;
 
     // The width/height of our scrolled area.
     LayoutRect m_overflowRect;
 
+    // ScrollbarManager holds the Scrollbar instances.
+    ScrollbarManager m_scrollbarManager;
+
     // This is the (scroll) offset from scrollOrigin().
     DoubleSize m_scrollOffset;
 
     IntPoint m_cachedOverlayScrollbarOffset;
 
-    // For areas with overflow, we have a pair of scrollbars.
-    RefPtrWillBeMember<Scrollbar> m_hBar;
-    RefPtrWillBeMember<Scrollbar> m_vBar;
-
     // LayoutObject to hold our custom scroll corner.
     LayoutScrollbarPart* m_scrollCorner;
 
     // LayoutObject to hold our custom resizer.
     LayoutScrollbarPart* m_resizer;
 
 #if ENABLE(ASSERT)
     bool m_hasBeenDisposed;
 #endif
 };
 
 } // namespace blink
 
 #endif // LayerScrollableArea_h