Hardening for the large-gallery stall. The reporter could not isolate the
cause and neither could I from static reading; these are two defects that
are wrong independently of whether they are the whole story.
Gallery grids are NOT virtualized: a 546-photo event puts 546 PhotoCards
in the DOM, each mounting its own bare fetch. Two problems there: no cap,
and a cleanup that only set a flag while the request kept running.
withImageFetchSlot now holds requests to six in flight, with the BODY read
inside the slot — fetch resolves on headers, so releasing there would have
bounded header round-trips and nothing else. Teardown aborts via
AbortController.
A request queued indefinitely is PENDING, not failed, which is why the
failure left no console error, no failed request and nothing in the
backend log.
Review follow-ups: three tiers (current lightbox slide > neighbour
prefetch > grid thumbnails), because a single FIFO put the image the user
just clicked behind hundreds of thumbnails; and a synchronous throw now
releases its slot instead of permanently draining the pool.
If it recurs, capture performance.getEntriesByType('resource') for the
stalled thumbnails — a queued request shows responseStart === 0.
Relates to #1287
Paul Nothaft
4afe7a6f08