Function lower_layer 

pub fn lower_layer(alpha: f32) -> LayerLowering

How a layer at alpha lowers.

A non-finite alpha answers LayerLowering::Isolated, the same as any alpha below one. It never reaches here from the frame path — the compiler’s up-front walk refuses such a layer outright — and the scheduler escalates a recorded layer whose opacity is not finite, so the conservative answer is the isolating one either way.