blurSizeRef property
Shapes whose half shorter side is below this get a proportionally smaller frost blur: σ = blurSigma × min(1, halfMin / blurSizeRef). 0 disables the scaling.
SwiftUI frosts small glass much less than large glass (measured in Task 17b from the coded captures: regular σ grows roughly in proportion to the shape's size). A group runs one blur, so it uses the largest member's σ.
Implementation
final double blurSizeRef;