ConstA large, touch-friendly proxy for a scrollbar thumb. It is a pure presentation of the thumb: it
rides the thumb's fractional position and reports drag back as a fraction, so the (virtual)
scrollbar owns all scroll math — including the elastic bounce, and (via onEdgeHold) the
accelerating extend when the finger parks past a travel end. The TouchThumbFrame published
by the enclosing ScrollPane tells it where to sit — inside the viewport, clear of the
perpendicular grip's virtual corner.
TouchThumbHandle component