Issue with Unity3D Canvas PixelPerfect, LayoutGroup and ScrollRect

Question

When the scene like below, the ScrollRect has no problem.

Canvas (Pixel Perfect toggle off)
--| ScrollRect ( ScrollRect + HorizontalLayoutGroup )
----| Content
----| Scrollbar

But when Pixel Perfect toggle of Canvas is on, the ScrollRect is not working.

Canvas (Pixel Perfect toggle on)
--| ScrollRect ( ScrollRect + HorizontalLayoutGroup )
----| Content
----| Scrollbar

And if remove the HorizontalLayoutGroup from ScrollRect, everything is back to normal.

Canvas (Pixel Perfect toggle on)
--| ScrollRect ( ScrollRect )
----| Content
----| Scrollbar

Hope someone can explain the reason behind it, thanks in advance.

I uploaded the test scene in Unity3D forum, hope it can help you to understand the problem.