A WebGL2 flicker fix (#1139) was render-parity-verified on exactly one scenario — the zoom-OUT hold-tiles case that reproduced the original flicker, retained-content 0.19 → 0.79 matching WebGPU — and never on zoom-in or per-frame cost. It shipped an unmemoized per-frame classifyTile run twice per tile plus synchronous uploads, and stuttered with progressively-black fills on a Seoul zoom-in. Reverted 36 minutes later (#1140). A hot-path fix must be verified in its worst regime, not the one that showed the original bug.
A draped landcover polygon rendered hundreds of kilometres offshore — but only on WebGPU. Every vector slice of one source shares one Material uniform pool, and each slice resets the pool cursor to 0, so a later slice's tile i overwrote an earlier slice's tile-i buffer. WebGPU's queue.writeBuffer defers to submit (last-writer-wins), so every draw read the final bytes; WebGL2's immediate bufferSubData never showed it — and the differential test was structurally blind, because both frames aliased identically.
Half the globe rendered as background again — same symptom as the drape-suppression bug, opposite root cause. This time the trans-antimeridian tiles were never selected: the overzoom fast-path unprojects the viewport corners to a lon/lat box, but on a small globe disc the corners miss the sphere, the box collapses to the sub-camera point, and it emits the single tile column under the camera — which a contiguous longitude range can never wrap across the dateline. Told apart from the draw bug by seam location and persistence.
A refactor rerouted the WebGPU frame shell through RHI wrappers and claimed byte-identical rendering. We did not diff pixels against a tolerance — we hashed three captures and got cd3097…9b0f three times. A same-code re-run proved the noise floor was zero, upgrading the gate from diff-below-tolerance to bit equality. Determinism is a property you engineer into the harness; once you have it, verification collapses from statistics to md5sum.
One debugging day, seven instrument failures: a probe that double-counted its own bookkeeping into '93 duplicate requests', a WebGPU canvas that reads back as all zeros, a parity test that passed by comparing two empty frames, a screenshot taken 7.5 seconds too early, and three more. Each with the tell that gave it away and the cross-check that caught it — because the same day also proved that one of those 'flakes' was hiding a real bug.
Half the globe rendered as pure background, cut exactly at the 0-meridian z1 tile boundary. The fallback ancestors for the missing half were computed, CPU-cached, and force-uploaded to the GPU — then drawn nowhere, because a drape suppression flag was scoped to the renderer instance instead of one dispatch. Verified by making the network slow on purpose, after the first pixel readback lied.
A row of country labels floated in the sky above the globe's horizon. Round 1 — an angular margin, provably safe at every zoom — died because floaters and healthy labels interleave in angular depth (Nigeria 0.332 floats, Kenya 0.329 doesn't). Round 2's screen-space limb fixed Chad and missed two-line Burkina Faso by exactly one line of text. The gate only held once it moved to where the quad height actually exists.
A headless session with no GPU had thirteen open issues and a hard rule against claiming a render is correct without a pixel diff. The split that worked sorted the backlog not by difficulty but by whether each fix's correctness reduces to something provable without a raster.
A concave country's label rendered offshore because the anchor was the bounding-box centre — a point the polygon does not contain. The fix is a guaranteed-interior point, but the harder call was shipping it from a session with no GPU to re-check the pixels.
A PR body cited a CI gate that no workflow references and a waiver clause absent from the issue it named — both written by an AI implementer, both refuted by one grep. Plus the real bug beside them, and the derived artifact a freshness gate caught.
Routing Earth constants out of GPU shader source through one planetary Body authority, under a hard bar: emitted bytes must not change. Goldens captured from pre-change code, a literal ratchet, a 211-hit census — and a "wrong" eccentricity kept on purpose.
I edited one file in @xgis/map and ran that package's whole suite — 1246 green. Then the full monorepo run turned up two failures, both in @xgis/runtime, a package I never opened. Mid-extraction, the sufficient gate is the whole repo, not the package.
A fix for per-feature fill opacity came with a fail-before/pass-after test: 0.25 and 0.75 baked to alpha 64 and 191, two values where there had been one. It was green and I reverted the change — because the alpha it proved feeds the outline, not the fill.
A single-authority migration for on-screen scale left the globe branch returning the raw, uncapped value under a comment reading 'UNVERIFIED'. A sibling branch of identical shape was a non-bug — and only a probe of which value is read told them apart.
Line strokes shook at deep zoom in non-Mercator projections: the shader rebuilt an absolute longitude in f32 degrees, then a later stage subtracted the camera again — catastrophic cancellation. The fix subtracts first; the proof needs no GPU.
We adjudicated eight open bug reports in one batch. Two were already fixed on main and one's premise was refuted by a write-only uniform lane. In every stale case the deliverable that survived was a regression test, not a patch.
GPU double-single splits floats with 8193; the textbook says 4097. I was sure 8193 was cargo-culted from a wider type — until two million float32 products came back error-free for both. The split constant that breaks is one too small; the real bug was never the split.
