Browser input is delivered exactly once by (clientId, seq). The server records a
watermark per clientId and discards anything not above it as a duplicate — but
acknowledged it with an ACK indistinguishable from "applied". The client then
dropped the record from its queue, the UI looked perfectly normal, and the
terminal received nothing at all.
The counter is persisted to localStorage through a debounced write. Kill the page
between "sent" and "persisted" and the restored counter is below the server's
watermark, after which every keystroke lands under it, is discarded, and is
ACKed. Reloading does not help: the clientId is restored from localStorage
alongside that stale counter. Measured on a real session — typing into the same
session from a fresh browser (new clientId, no watermark on the server) worked
perfectly, which is what localised the fault to client state.
Three changes:
- on rejection the server replies {"t":"ia",seq,"dup":true,"last":<watermark>}.
It still ACKs, so the client can drop the record from its queue, but it now
says the input was not applied and supplies the number needed to climb out.
- on `dup` the client lifts its counter above the watermark and re-queues.
⚠️ Only records whose FIRST delivery is being retried are re-sent: a retry
judged duplicate means the mechanism is working (the original did arrive), and
re-sending would type the same text twice.
- the counter is now persisted synchronously. The queue payload can stay
debounced, but the counter is the thing that has to survive a crash, and
leaving it on the lossiest path cancels the only guarantee there is.
⚠️ Reading the watermark is defensive: the session arrives through a structured
port, and a port missing that method must not take the whole input path down —
a throw inside the handler means the ACK is never sent and the record is stuck in
the client queue forever, which is worse than the ambiguity being fixed. A mock
port's test timeout is what exposed this.
(cherry picked from commit 05bb7081cc)
