a0a3c716f fixed the exact same failure mode for Telegram (#58563):
post-#48648, oversized mid-stream edits truncate to a one-message preview
instead of splitting. Once a long streamed reply grows past that cap, every
subsequent progressive edit truncates to the SAME preview text — re-sending
an identical edit every tick still counts against the platform's edit rate
limit for the rest of the stream.
Discord's edit_message() has the identical architecture (mid-stream
truncate-in-place, both pre-flight and reactive-after-50035 truncation
paths) and this file's own docstring already calls out "the Telegram #48648
lesson" it's built on — but the saturated-preview dedup fix itself was never
ported over.
Fix: track the last truncated preview per (chat_id, message_id), mirroring
a0a3c716f exactly. Skip the edit call when the new truncation is identical;
still deliver when the visible content actually changes (e.g. the
chunk-count marker crosses (1/2) -> (1/3) as the stream grows). State
clears on finalize and when content shrinks back under the cap, so dedup
can never mask a real edit.
2e2212be1b