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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.h b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.h
index bdbf1bbba9fb0174f020b1262cb4f88eeb94017c..41f03875d70c4a3780b3ce45c10e44ac50362fa0 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.h
@@ -33,6 +33,7 @@
 
 #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"
@@ -41,7 +42,6 @@
 
 namespace blink {
 
-class FloatPoint;
 class ScrollableArea;
 class Scrollbar;
 class WebCompositorAnimationTimeline;
@@ -71,6 +71,7 @@ public:
 
     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.