It does both (infinite scrolling is a horrible idea). If you try dragging the scrollbar it will jump around every time more content is loaded and you lose your position on the page.
Apart from the slow pace at which the page scrolls back, the fact when it's doing so you cannot scroll up anymore (or down when you're at the bottom of the page) annoys me even more.
What Apple did, making scrolling behave like a sheet of paper confirmed using a rubber band, seems more natural than what this "3d layout engine" is doing. Giving it even more thought, it makes sense that even on a non-scrollable page scrolling (giving some form of input) should at least cause a confirmation that it recognised the action you preformed.
I also noticed that on my phone (Nexus 4) sometimes the scrolling just keeps going. I'm not sure that's how they meant it to behave, but even if it is, it's not great.
This doesn't seem to be designed with regular desktop usage in mind.