# Thumbnails sized for the tile's width at 2x, not 512 on the long side

_2026-09-23. Operator on sindra-nude-final: "the images look blurry until
they're selected and blown up." Commits `c2b1454`, `c19d8c9`, merged
`1d31ab0`._

## The measurement

- The cap was 512px on the LONGEST side. Tiles are sized by WIDTH (`width:100%;
  height:auto`).
- Chromium put desktop tiles at 321-361 CSS px (3 columns, 1440px viewports
  and up), 324-472 at 2 columns, and up to 650 at 1 column (measured across a
  360-2560 viewport sweep).
- A 704x1408 portrait got a 256-wide thumbnail: stretched 1.4x at 1x density
  and 2.8x on a 2x screen.
- The live sizes are mostly 704x1408, 896x1216, 832x1216 and 1024x1024
  portraits (368 images sampled).

## The rule

- `THUMB_WIDTH = 768` (2x the widest desktop tile) and `THUMB_HEIGHT_MAX =
  4096`.
- An original that fits is served as-is only when it's also light (<=64 KB;
  768-wide thumbnails average 39 KB) or animated.
- `tests/test_thumbs_browser.py` binds 768 to the rendered grid at 1440, 1920
  and 2560.
- Cost across all 381 live images: 4.8 → 14.2 MB at 768; 1024 would have been
  18.5 MB. Live rebuild: 14.5 MB, 369 webp and 12 originals, 9 s.
- **The operator kept 768 (2026-09-24).**

## heid bug-hunt (4/4, five seat-executed probes) → `c19d8c9`

- A cache hit must be a REGULAR file with its source's EXACT mtime. A planted
  directory was served as the thumbnail, and `cp -p` pinned stale ones
  forever.
- The cache dirs are made component by component without following links. A
  `.thumbs` link put the cache outside the booth.
- The temp file is mkstemp. `<out>.<pid>.tmp` was plantable as a link: 600 B
  became 316,400 B.
- Palette transparency survives. This one was INTRODUCED by `c2b1454`'s
  fits-but-heavy branch.
- EXIF orientation is honoured.
- A 64 MP decode budget.
- The whole rule is in the cache name: `.768x4096q78v2.webp`.