After a turn completed on a long conversation that loaded truncated, the
viewport jumped to the first message. The done SSE handler replaced S.messages
with the full transcript payload and updated _messagesTruncated, but did NOT
reset _oldestIdx the way the canonical full-load paths do (sessions.js
_ensureMessagesLoaded, ui.js loadSession).
The read-position rework (#4613) keys the scroll anchor on an absolute index
(sessionIdx = _oldestIdx + rawIdx; rows tagged data-session-msg-idx). On done
the handler also expands the render window from the tail window (50) to all
messages, shifting every rawIdx. With _oldestIdx left stale, the captured
anchor resolved to an earlier row and the fallback used the raw scrollTop
against the now-taller full transcript, landing near the first message.
Reset _oldestIdx from d.session._messages_offset||0 alongside _messagesTruncated.
The done payload (_session_payload_with_full_messages) embeds the full transcript
and carries no offset, so this resets to 0 and keeps the absolute anchor aligned
with the rebuilt rows. No-op for non-truncated sessions; pinned-at-bottom readers
still follow to bottom (shouldFollowOnDone path unchanged).
Both gates clean (Codex SAFE TO SHIP, Opus ship-it) — verified the done payload
is always a full transcript with no offset, and every other _oldestIdx consumer
becomes more consistent.
