frostWideSigma property
Sigma of the wide frost tail added in the shader on top of the blurSigma core (logical px; 0 disables the tail).
SwiftUI's frost is a sharp core plus a wide tail (Task 17c, measured
with five code periods by tool/fidelity/measure_frost.py):
(1 − w)·G(core) + w·G(core ⊕ tail). The composed blur is the core;
the shader samples the blurred texture 16 times on two rings (a 2-node
Gauss–Laguerre rule for a 2-D Gaussian of this sigma) and mixes that in
by the weight w of frostWideMixEdge.
Implementation
final double frostWideSigma;