After a hidden interval that added persisted messages (post-turn bg-review
writes, sibling-tab writes, the just-finished main turn flushing), switching
the tab back caused refreshActiveSessionIfExternallyUpdated('visible'|'focus')
to hit remoteCount !== localCount and call loadSession(sid, {force:true}),
which synchronously did S.messages = [] before awaiting the metadata +
messages fetches. The visible result was the entire conversation transcript
blanking for the round-trip and reappearing — '对话突然消失，重刷才回来'.
sessions.js itself warns about this 'disappear and reappear' tradeoff at the
top of the remoteCount/remoteLast guard, but only short-circuits the
metadata-only branch (#5061). #5122's 4-probe staged reconnect covers a
different path (SSE error with ready_state=2 on a visible tab) and does not
apply when the SSE error arrives while visibility_state='hidden' — that
bottoms out through _deferStreamErrorIfPageHidden before the reconnect block
ever runs.
Fix:
- refreshActiveSessionIfExternallyUpdated maps the visibility/focus recovery
reasons ({visible, focus}) to a new keepStaleUntilLoaded option, forwarded
to loadSession. The post-stream idle reconcile and the poll/external paths
keep their existing behaviour.
- loadSession ANDs opts.keepStaleUntilLoaded with sameSessionForceReload (so
cross-session switches still clear synchronously — leaving a prior
session's transcript on screen during a navigation is the original bug
the clear was written for) and, on that path, skips the synchronous
S.messages/S.toolCalls/_messagesTruncated/_oldestIdx clear. The new
transcript is SWAPPED into S.messages by _ensureMessagesLoaded(sid,
{force:true}), so the user sees old DOM directly replaced by new DOM in a
single render frame.
- _ensureMessagesLoaded grows an opts.force escape hatch so its 'messages
already populated' early-return cannot skip the swap when stale messages
are still in place.
Verified live on a source 8701 build:
- before: minHtmlObserved 65874, minKidsObserved 11 — 372 samples at 16ms,
DOM html length NEVER dipped below the pre-call value during the reload.
- loadSession was called with force:true keepStale:true reason:'visible',
outcome 'reloaded', sMsgs 21 → 1037, after kids 11 → 134 (single-frame
swap).
Tests:
- tests/test_issue5177_hidden_tab_blank_gap.py (new, 7 cases): source-locks
the keep-stale guard shape, the recovery-reason map, the
_ensureMessagesLoaded opts.force escape hatch, and that the synchronous
clear is wrapped in if(!_keepStaleUntilLoaded).
- Updated the two existing source-lock tests whose anchor strings included
the loadSession call signature
(tests/test_webui_external_refresh_frontend.py +
tests/test_tars_scroll_reset_regressions.py) to match the new
loadSession(... keepStaleUntilLoaded:_keepStaleUntilLoaded) form. No
behavioural assertions changed.
Local: node --check static/sessions.js OK. 122 passed across
test_issue5177_hidden_tab_blank_gap.py + the full adjacent set
(external-refresh / journal-frontend / scroll-reset / reconnect-chronology /
inflight-stream-reuse / sse-error-multi-probe / issue4295 / issue4856).
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