The list endpoints deliberately back-fill pinned conversations past their
LIMIT, then the client sliced the response back down to that same limit
and threw them away — so only pins that happened to land inside the most
recent page ever rendered, which reads as a cap on how many sessions you
can pin.
Keep the back-filled rows when trimming, and discount them from the
"window came back full" test that drives Load more. Counting a back-fill
as a loaded row invented a page that could never be fetched, leaving a
Load more button that refetched the same rows forever.
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