Function position_fraction 

pub fn position_fraction(list: ScalingList<'_>) -> f32

How far down the track the thumb’s leading edge sits, before the thumb’s own length is taken out of the travel. 0.0 at the top, 1.0 at the bottom.

The denominator is the number of items that are not on screen — how far the list can still travel, counted in items — which is why this is not the same number as a pixel scroll’s progress on a list whose rows differ in height.