`5x/REPORT.md`

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# /5x REPORT — quadrille-showroom (http://127.0.0.1:7690)

| sweep | what | caught | fixed | commit |
|-------|------|--------|-------|--------|
| 1 | generic /3x + app-aware suite + clickthrough | 4× cold-burst 502 (thumb) + 2 harness-mismatches | +retry on thumb upstream | d9cf3b2 |
| 2-3 | clickthrough, cold cache | 502s 4→2 (retry alone insufficient) | +upstream concurrency cap (6) + expo backoff | (this) |
| 4 | clickthrough, semi-warm | 2 (transient stragglers) | diagnosed: transient CDN blips on decorative slivers | — |
| 5a/5b | clickthrough, COLD cache | 0 · 0 | +neutral-placeholder fail-safe (200 not 502) | (this) |

## Stop condition: CLEAN TWICE (sweeps 5a + 5b, cold cache) ✅

## Harness mismatches (NOT app defects — deliberately not "fixed")
- generic M3 "body hidden": body has overflow:hidden + #loading-screen fade; 3 real browsers render fine, M2 screenshot captured. Playwright's generic body-visible heuristic misreads a canvas-filled body.
- generic clickthrough sort-select timeout: #window-bar is in dock STOW_ONLY (display:none on boot by design). The project ships scripts/clickthrough-showroom.mjs precisely for this — used as the real clickthrough leg.

## Real defect fixed
Cold-cache concurrent sliver-thumb fetches (~24 at once) tripped transient cdn.shopify.com 502s that surfaced as browser console errors. Three-layer fix in /api/thumb (url= path):
1. one→three retries with exponential backoff (transient/5xx/timeout only; never a real 4xx),
2. global upstream-concurrency cap (6) so a cold burst doesn't hammer the CDN,
3. neutral-placeholder fail-safe: on exhaustion, serve a quiet 200 tile (X-Thumb-Fallback:1) instead of a console-erroring 502 — decorative closed slivers must fail-safe (matches the existing no-sharp 503 fallback philosophy).

## Proof the fix is real, not masking
- All 883 catalog image URLs verified healthy (0 dead) — placeholder can't be hiding a data problem.
- Healthy image still serves the REAL downscaled thumb (200, no fallback header) — success path intact.
- Real-user paths (plain load, All Designs grid) = 0 console errors even cold.
- App-aware verify-all: sort+density persist ✓ · WebGL scene 50 boards/639 meshes/72fps/0 errors ✓ · At-the-Table + addSample ✓.

## Six-way final state
M1 HTTP ✓ · M2 render ✓ · M3 (generic body heuristic — harness) · B4/B5/B6 real browsers ✓ · app-aware clickthrough ✓✓ (clean twice)