The real reason every thumbnail card stayed on the placeholder was a
reference-aliasing bug in the flush closure. `const batch = pendingUrls`
captured the same object; `delete (pendingUrls)[k]` for each key then
emptied `batch` too, because they were the same reference. By the time
`setThumbnailUrls(prev => ({ ...prev, ...batch }))` ran, batch was {}
and the state never picked up any URL — every card rendered the empty
placeholder icon.
Fixed by aliasing first, then replacing pendingUrls with a fresh empty
object (let instead of const on the outer binding). The cloned `batch`
retains its entries for the spread; any new arrivals during the commit
land in the new empty pendingUrls and coalesce into the next frame.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
byGalax
e2e8217b86