Function band_depth 

pub fn band_depth(extent: f32, factor: f32, floor: f32) -> f32

The depth of one edge band along the axis it is measured on: the caller’s fraction of the extent, raised to floor when that fraction does not reach it.

A fraction alone cannot answer this. zone_size_factor is one number for both axes, and it is the shape the caller wants — a fifth, a quarter, a bisection — so on a large target it is right and must not be touched. On a small enough one, any of those fractions is a band no finger can land in, and the drop it swallows goes to the neighbouring zone with no warning. So the fraction wins wherever it already conforms and the floor takes over below that, which is the same shape as dp — a floor that only ever raises, leaving every target that was already big enough exactly as it was.

The floor itself is capped at a third of the extent, for the reason partition_targets splits evenly when its own floor cannot be met: on a target too small for leading | centre | trailing at the floor, three equal bands keep every zone reachable and visibly sub-floor, where an uncapped floor would let two opposing bands meet and delete the centre.