The button's visibility mixed the sync meta with the prefetch-cache hint;
a stale prefetch entry (cursor recorded at the initial page) could keep
the affordance alive after the user had already loaded to the top. Sync
now exposes an explicit isComplete (positive confirmation from a fetch,
distinct from !hasMore on unpopulated meta) and it overrides the prefetch
hint, which stays in effect only before the meta knows anything.
Bohdan Triapitsyn
01ca2ecf24