Root causes found via per-event tracing (trace.log) + heartbeats:
1. Decode backlog: single worker at ~300ms/photo could not keep up with
fast long scrolls — phone session ended with a 2400-deep queue
draining at ~3 cells/s (13+ min to catch up). That was the
'scroll far down and nothing loads no matter how long I wait' zone.
- 4 parallel decode workers (~12 photos/s)
- viewport-aware stale drops: prefetch jobs more than 60 rows behind
the viewport are dropped, not decoded (fleeing leaves no backlog)
- Q_CAP bounds the queue; JS 1.5s safety net re-requests
- queue peak in repro test: 42 (was 2405); all 850 decodable
emulator images now load (708 before)
2. Stale-anchor bug: sendAnchor was leading-edge throttled with no
trailing call — if a scroll ended inside the 150ms window the Rust
anchor stayed stale forever, and the worker then dropped fresh
URGENT jobs (tap → full read) as 'off-screen'. Fix: trailing
anchor send; and URGENT jobs are never stale-dropped (only
prefetch work is droppable).
3. Viewer latency: full-res read ran on Chromium's main thread
(wry shouldInterceptRequest is synchronous) — 30-50MB flash read
froze the whole WebView. Now: thumb shown instantly (mem cache,
~1ms), full read queued urgent and done off-main; full-ready event
swaps in the full-res (256MB full-res LRU makes re-opens instant).
Measured click→full-res: 4ms on emulator.
4. Scan: skip OS junk (._* AppleDouble forks, dotfiles) — the phone's
DCIM carried 72 of them, rendering as permanent gray cells.
5. Viewer for a corrupt file now shows 'image unavailable' instead of
'loading…' forever; grid retry timers skip far-off-screen cells.
Greg Pomerantz
02d1f097ac