The web build black-screened on the first frame: the black-hole fragment
shader failed to compile in Chrome's Tint (BrowserWebGpu backend) with
"'textureSample' must only be called from uniform control flow", which
invalidated the opaque_mesh2d_pipeline and made Bevy quit after frame 1.
Desktop (naga) was unaffected because naga does not enforce this WGSL rule,
which is why the regression was web-only.
skybox_color() called textureSample on the cubemap from inside the main RK45
integration loop. That loop's control flow is non-uniform — per-pixel early
exit via `if (accum_alpha > 0.99) { break; }` and per-iteration accept/reject
— and textureSample needs screen-space derivatives (uniform across the quad)
for mip selection, so the spec forbids it there.
Switch to textureSampleLevel with an explicit LOD of 0. textureSampleLevel
does no derivative computation and is permitted in non-uniform control flow.
LOD 0 is correct for the skybox: it is sampled at full resolution with no
mip minification, so the visual result is identical to the implicit-lod path.
Diagnosis required building a JS-side probe that fetches the WGSL, mirrors
naga_oil preprocessing (resolve #{MATERIAL_BIND_GROUP}, inline VertexOutput),
then calls device.createShaderModule + getCompilationInfo to surface Tint's
structured messages — Bevy's wgpu path swallows the raw Tint error on web and
only ever surfaces the downstream "Invalid ShaderModule due to a previous
error". The probe confirmed the error site and rule; it is not committed.
xfy
5c0b1db01b