A translation is scaffolded from both ends now -- the chapters that existed when
it was proposed, and each chapter signed afterwards putting itself in. Neither
end closes the gap on its own, and no snapshot taken at proposal time can.
Two ways they miss each other. A chapter and a translation proposed at the same
moment each read what exists when they are proposed, so neither sees the other
and nothing retries. And a scaffolding session that fails to reach a quorum
leaves nothing behind to try again with -- the chapter's id is spent, since a
re-proposed chapter is a different one.
So the translation's own screen says what it is missing and offers to ask for
it. Above the chapter list rather than below, because a chapter that is not in a
translation is invisible from a list of the ones that are: the whole failure is
that nothing looks wrong.
**Matched on the chapter named, not counted.** `chaptersMissingFrom` compares
which source chapter each translation chapter stands for. A count would read a
translation that is missing its second chapter but picked up its third as one
missing its last, and would then scaffold the wrong chapter -- leaving the real
gap open and a duplicate beside it. It also means running a catch-up on a
translation that is already complete proposes nothing at all, rather than a
second copy of every chapter under fresh ids.
**More sessions rather than a cap.** The missing chapters are chunked at
MAX_BATCH_SIZE, one session each. A translation far enough behind to need more
than a batch holds is not one to refuse; it is one the group answers for more
than once. They share a timestamp, so a catch-up reads as the one act it is.
The screen lands on the first session -- the rest are beside it in the room's
list -- and the card stays until a quorum arrives, which is honest: the chapters
are still missing until then.
**Tests.** Two more in TranslationScaffoldTest, both about the matching rather
than the counting: a gap in the middle, and a complete translation being missing
nothing. Checked against a broken implementation -- taking the missing chapters
as the tail after a count passes on a translation that fell behind at the end,
which is the easy case, and is caught by the gap.
Not covered: `catchUpMissingChapters` itself, which is plumbing over the
templates those tests pin and the batch API the jvm tests pin.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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