Five smoothstep edges reversed, and SwiftShader folds them to a constant: where the rain went

This is the best debugging record in the section, because the bug it hit only shows up on one particular implementation.

Six render rounds, each checking ready, demoError, page and console errors, __DEMO_INFO, pixel statistics, det=1 determinism, palette switching via key=2 and key=3, and a free-running render with no t:

1. A TDZ bug: a let declared after init() ran. Fixed by hoisting.
2. Washed out at mean luminance 0.59. Rain alpha, bloom strength and threshold, window brightness and exposure all reduced.
3. – 4. Found where the rain went. Five smoothstep calls had their two edge arguments reversed, and this SwiftShader build evaluates an edge-reversed smoothstep as a constant. It confirmed the symptom with a debug copy that forced point size and alpha, then rewrote every instance as 1.0 - smoothstep(lo, hi, x).
4. – 6. Raised ambient and fog so no frame is dominated by black pixels, spread the searchlights along the flight path, and confirmed a full beam at t=66.

Final: mean luminance 0.13–0.21, over 93% of pixels non-black, two identical hashes at t=7.5, about 3 FPS under software rendering.

The bug in rounds 3 and 4 is the strongest argument in this set: it is neither a syntax error nor a logic error, and on a conforming implementation it might work fine. No static check reports it, and no amount of the model “thinking it through” avoids it — the only route is to render the frame, notice the rain is gone, and write a debug copy to isolate it.

The scene itself is heavier than the Astra run: 5,422 buildings from a single InstancedMesh with a fixed-seed layout, streets every fifth cell, a 60 m flight corridor carved along the camera path and a taller downtown core; windows, random flicker, neon sign bands, edge strips and rooftop beacons all drawn in the fragment shader with distance LOD; wet ground sampling a half-resolution reflection render target with flowing distortion, two layers of expanding rain rings, a puddle mask and Fresnel. Higher numbers — but framed from above and dark overall, so the composition reads less like a city than Astra’s does.

Controls: mouse movement for parallax, scroll to zoom the view, space to pause, 1/2/3 for palettes (Neon Noir and others), and dragging gives a temporary free look that returns to the flight path on release.