ProgressiveBlur class

A progressive (graduated) backdrop blur — the Signal / iOS-26 header look: a clean gaussian frost that is strongest at one edge and eases to perfectly sharp at the opposite edge. Stack it behind a translucent app bar so content dissolves beneath it instead of ending on a hard cut-off.

This is the graduated-blur primitive the rest of the library does not provide (glass surfaces apply a uniform blur). It is self-contained — it needs no LiquidGlassLayer or glass ancestor — so it can back any bar.

How it works — a single GPU pass that samples the backdrop

The naive "blur then fade with a ShaderMask" recipe does NOT work: a BackdropFilter's captured backdrop is not included in an ancestor ShaderMask's layer on Impeller, so the mask reveals nothing and iOS shows no blur at all. Instead this uses ui.ImageFilter.shader: a fragment shader runs as the ui.ImageFilter of a BackdropFilter, so the engine binds the captured backdrop to the shader's sampler — sampling it reliably on every backend. shaders/progressive_blur.frag reads that backdrop with an importance-sampled gaussian whose sigma follows the gradient (normalised over the widget's own device-pixel rectangle, since the bound texture is the whole screen), giving a smooth, band-free dissolve in one backdrop capture + one draw.

Drive maxSigma from a scroll offset to fade the blur in/out (0 → sharp).

Stack(
  children: [
    const Positioned(top: 0, left: 0, right: 0, height: 96,
      child: ProgressiveBlur(maxSigma: 20)),
    // ... your translucent app bar on top ...
  ],
)

Call preload once from main() (after the binding is initialised) to pre-compile the shader so the first bar paint already has it.