The grid already showed the camera's embedded preview first, then queued a render of the whole raw
to replace it. For a 512 px thumbnail that render decodes 50 of a 56 MB Fuji RAF's bytes for a
picture the 4 416 × 2 944 embedded JPEG already covers — 45 s a photo on the 1.2 MB/s share this was
measured on, 8.3 GB of traffic for a 149-photo folder, and a queue that delayed the render the user
was actually waiting for (measured: the first loupe render of a cold photo finished 174 s after the
click, most of it spent behind thumbnails). Idle for a minute afterwards, the same folder read 12
previews (26.9 MB) and decoded zero raws for thumbnails, against 638 MB of whole-file reads before.
- `Session::embedded_thumb_is_final` answers whether a photo is an unedited raw whose camera JPEG the
viewer is showing — the same `has_import_look` gate the preview path uses.
- `grid::skips_the_real_render` is the decision (unit-tested): a cached render as stand-in, an edited
photo and a raw without an embedded JPEG all still render, because their thumbnail must show the
pipeline's output rather than the camera's.
- `Renderer::remember_embedded_thumb` records the quick render's key and the bucket, and
`thumb_quick_is_final` / `thumb_quick_pending` let the per-frame grid request stop at the embedded
preview instead of rebuilding the render job (and its cache key) every frame.
- The thumbnail key still follows the settings hash, so editing a photo re-renders its thumbnail
through the ordinary background path.
`docs/raw-preview-reads.md` and the `LR-IMP-PREVIEW-READ` parity row record the behaviour and the
numbers.
Co-Authored-By: DeepSeek V3.2 <noreply@deepseek.com>
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