Function indicator_segments 

pub fn indicator_segments(
    arc: IndicatorArc,
    geometry: IndicatorGeometry,
    alpha: f32,
) -> [(IndicatorPart, IndicatorSegment); 3]

The whole indicator as a list of drawable pieces: track, thumb, track.

It is three separate segments with a gap at each end of the thumb, not a thumb painted over a continuous rail — drawing a full-length track under a thumb gives a visibly different picture where the gaps should be.

alpha scales every piece, which is how the indicator fades out after the list has been still.