The pre-load band made tiles arrive in time. It did nothing about them never
leaving. PhotoCard latched its observer with triggerOnce, so a tile that had
been scrolled past stayed mounted for the life of the page — holding its
object URL, and where image protection is on a canvas sized to the image that
the browser is not permitted to evict.
Measured in Chrome on a seeded 546-photo grid, scrolling top to bottom:
mounted tiles climb 24 → 100 → 212 → 364 → 546 and never fall. That is a
monotonically growing retained set, which is the profile a memory-constrained
browser discards a tab over — the reported symptom on iOS Safari 18.1 being
tiles that stop appearing and a blank page after refresh. With this change the
same scroll peaks at 68.
PhotoCard now takes an optional outer band. The inner band, unchanged, decides
when a tile starts loading; the outer one decides when it is far enough away
to unmount, and unmounting is what actually frees anything, because
AuthenticatedImage revokes its object URL and drops the canvas in its cleanup.
The gap between the bands is the hysteresis: at three viewport heights against
a one-viewport load band, a tile travels two further viewport heights after it
stops loading before it is released, so ordinary scrolling never crosses both
edges. Thumbnails are served private, max-age=1800, so returning costs a cache
hit rather than a round trip.
Opt-in per layout, and only Grid opts in. Its skeleton is aspect-square and
holds the tile's box exactly, so releasing shifts nothing; the measured
layouts have no such guarantee. Without the prop the observer keeps its
original latch, so every other layout behaves exactly as before — pinned by a
test, since that is the half most easily broken by accident.
This is not presented as the fix for the iOS report. It removes the mechanism
that best explains it, and it is worth having on any device; whether it is the
mechanism still needs a measurement from the phone that failed.
Relates to issue 1287
Paul Nothaft
a6a1db5254