Fix smooth scroll overshooting when mouse held down in scrollbar track.

third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.h

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 15 matching lines @@
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef ScrollAnimatorBase_h
 #define ScrollAnimatorBase_h
 
 #include "platform/PlatformExport.h"
 #include "platform/PlatformWheelEvent.h"
+#include "platform/geometry/FloatPoint.h"
 #include "platform/geometry/FloatSize.h"
 #include "platform/heap/Handle.h"
 #include "platform/scroll/ScrollAnimatorCompositorCoordinator.h"
 #include "platform/scroll/ScrollTypes.h"
 #include "wtf/Forward.h"
 
 namespace blink {
 
-class FloatPoint;
 class ScrollableArea;
 class Scrollbar;
 class WebCompositorAnimationTimeline;
 
 class PLATFORM_EXPORT ScrollAnimatorBase : public ScrollAnimatorCompositorCoordinator {
 public:
     static PassOwnPtrWillBeRawPtr<ScrollAnimatorBase> create(ScrollableArea*);
 
     virtual ~ScrollAnimatorBase();
 
     virtual void dispose() { }
 
     // Computes a scroll destination for the given parameters.  The returned
     // ScrollResultOneDimensional will have didScroll set to false if already at
     // the destination.  Otherwise, starts scrolling towards the destination and
     // didScroll is true.  Scrolling may be immediate or animated. The base
     // class implementation always scrolls immediately, never animates.
     virtual ScrollResultOneDimensional userScroll(ScrollbarOrientation, ScrollGranularity, float step, float delta);
 
     virtual void scrollToOffsetWithoutAnimation(const FloatPoint&);
 
     virtual void setIsActive() { }
 
 #if OS(MACOSX)
     virtual void handleWheelEventPhase(PlatformWheelEventPhase) { }
 #endif
 
     void setCurrentPosition(const FloatPoint&);
     FloatPoint currentPosition() const;
+    virtual FloatPoint desiredTargetPosition() const { return currentPosition(); }

[jbroman, 2016/01/19 22:39:12]

It's not obvious to me why this definition makes sense for ScrollAnimatorMac.

[skobes, 2016/01/19 23:44:14]

The bug is specific to the non-Mac scroll animator, so this change preserves the
existing behavior on Mac.  I don't think ScrollAnimatorMac even stores the
target, so it wouldn't be useful for it to override this.

 
     // Returns how much of pixelDelta will be used by the underlying scrollable
     // area.
     virtual float computeDeltaToConsume(ScrollbarOrientation, float pixelDelta) const;
 
 
     // ScrollAnimatorCompositorCoordinator implementation.
     ScrollableArea* scrollableArea() const override { return m_scrollableArea; }
     void tickAnimation(double monotonicTime) override { };
     void cancelAnimation() override { }
@@ skipping 38 matching lines @@
     float clampScrollPosition(ScrollbarOrientation, float) const;
 
     RawPtrWillBeMember<ScrollableArea> m_scrollableArea;
     float m_currentPosX; // We avoid using a FloatPoint in order to reduce
     float m_currentPosY; // subclass code complexity.
 };
 
 } // namespace blink
 
 #endif // ScrollAnimatorBase_h