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.
I spec'd a drift guard for the globe's camera-relative ECEF offset: force the ellipsoid constant to a sphere, assert the result shifts >10 km. The implementer measured it: 0.7 m. Turning the WHOLE computation spherical barely moves a relative offset, because both endpoints carry the same 21 km bias and a subtraction cancels it. The regression that actually bites is the mixed frame — ellipsoid tile, spherical camera — and only a guard that breaks the symmetry the same way sees it.
A building went missing from the 3D layer. Tile clipping had split its footprint into two pieces; the tiler flattened both outers into one rings array, and the extrusion consumer read everything after rings[0] as a hole — so piece #2 was earcut-punched out of piece #1's roof. Walls survived, roofs vanished, n pieces rendered n-1 roofs. The 2D fill was pixel-perfect the whole time, and zero tests had ever fed the consumer more than one polygon.
We broke instanced batching on purpose to watch a brand-new CI gate go red — 100,000 draw calls — before trusting its green. Why new gates need a demonstrated failure, and why this one asserts dc(10k) === dc(100k) instead of pinning an exact count.
The same render test passed twice: once at 38.5% painted pixels on real WebGPU, once at 93.6% on a silent WebGL2 fallback the assertion never noticed. Why a designed-in fallback chain turns every green output test ambiguous — until the test also pins which backend produced the frame.
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.
Two overlapping map labels swapped survivors on pan because the collision pass's tie-break was tile-dispatch order — reversed. The fix is a stable identity fed to the greedy allocator, and the gate is a permutation test.
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.
The verification stack behind shader-dsl's fp64: a correctly-rounding f32 machine built from Math.fround, a metamorphic oracle gate, byte goldens, and discriminative real-GPU known answers.
We rebuilt ten classic ShaderToy effects in X-GIS's typed shader DSL. The renders were the easy part: authoring them surfaced eleven concrete DX issues — an elliptical sun no gate could see, a reversed smoothstep every GPU hides — each closed with a gate so it stays fixed.
Our CI runs real shaders on SwiftShader — but only some render claims survive a CPU rasterizer. The dividing line we use (compute parity and shader compilation: yes; full-pipeline pixel parity: no), and the red-baseline incident that taught us to blame the software GPU last.
Three rounds of CI failures where every test was green and the job was red — vitest's worker→main reporter RPC timing out under file-count pressure. The ~110-file floor we split on (set below the lowest count we watched fail), the matrix design that contains it, and the playbook for the next regrowth.
Three real bugs from making WebGL2 match WebGPU on a 117-layer basemap: a dedup map nobody cleared, a depth jitter that out-voted painter's order, and a vertically mirrored composite hidden by a symmetric test fixture.
Our CI container's browser can't reach the internet, but the final render gate needs a real 117-layer basemap. On mirroring styles, tiles, glyphs and sprites locally — and the traps: SPA fallback serving HTML as tiles, unencoded fontstacks, and settling on signals instead of sleeps.
