I think it is interesting that you define a handle, but don't listen for events on it.  I would have expected a touch-and-drag on the handle to adjust the zoom.  Or any tap elsewhere on the container to switch immediately to that zoom level.

If I'm understanding the code right, if the zoom level is at 1, and I touch the zoom bar up near the top and move up, the thumb will move up a little bit as I move my finger, but it will not move to where my finger is.   I think we should check that with UX.