fix(terminal): skip a bounded Shell window before the downgrade guard

A window cut at the tail size can be smaller than the browser's buffer
while tmux still holds more. The downgrade guard reads that as "tmux has
nothing more to give", which is true of an unbounded capture only, so a
bounded window reaching it marked the session exhausted and removed Load
full history from the banner.

The bounded skip now runs first, so such a window never reaches the
exhausted path, and it no longer writes banner state: relabelling it from
the bounded payload would call a terminal holding all of a Load full
history pull "the most recent 1 MiB".

A skipped window that came back truncated cannot reach anything older
than the browser shows, and every ask costs the server a synchronous
capture-pane of the whole history (tail is applied after the capture), so
it puts the session on the 60 s cooldown. An untruncated one keeps 4 s.

_replayWouldShrinkBuffer takes optional pre-estimated rows so a megabyte
capture is not scanned twice. CLAUDE.md's Full-scrollback replay entry no
longer says Shell never pulls on ordinary scroll.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -205,7 +205,7 @@ Further detail: the `<prefix>: <title>` form (`w3-myapp: fix the login redirect`
 
 ### Full-scrollback replay
 
-**Full-scrollback replay** (COD-164/#148, reworked for #205): `GET /api/sessions/:id/terminal?full=1` returns the ENTIRE tmux scrollback (capture-pane `-e -S -<lines>` bounded by the configured history limit, explicit `maxBuffer` from the terminal-history config, early byte-cap before normalization, CRLF-normalized for shell panes). On success the capture is returned ALONE (`source='mux-full-history'` — it supersedes the byte buffer; no duplication). The first load of each non-shell TUI session per page requests `full=1` (`_fullHistoryLoaded` Set in app.js — the old one-shot `_initialFullBufferLoad` flag was consumed by whichever tab auto-selected, leaving every other TUI tab one frame of history). Shell sessions instead load a bounded 1 MiB `?tail=` window on every selection and automatic drop recovery: a 100k-line shell capture can be tens of MiB, and automatically parsing it makes tab-switch latency scale with the entire session. Shell FULL history is explicit-button-only; reaching the top during an ordinary wheel/touch gesture must not reset xterm and replay the multi-megabyte capture on its main thread. That gesture instead pulls a BOUNDED window of it (`?full=1&tail=<TERMINAL_TAIL_SIZE>`, so a capture beyond 1 MiB comes back `truncationReason: 'tail'` and the banner offers the rest), and skips the replay when the window holds no more rows than the browser already has. Declining the gesture outright was a dead end: after any burst a shell pane held about one screen of browser scrollback, and the button renders only once a replay was truncated, so a young shell tab had no way back to history tmux still held. Other modes may still re-pull `full=1` at the TOP, and pressing **Load full history** forces the request for any recoverably truncated session (`_maybeRefetchFullHistory`, 4s per-session gesture cooldown, in-flight + tab-switch guards, viewport position held across the replay); Shell full pulls are not retained in the tab cache, so the next switch stays bounded. Chunked replay enqueues 32 KiB pieces across safe yields, appends an xterm parse marker, then releases the live-output gate; output arriving after that release stays ordered behind the snapshot, and the marker callback supplies accurate parse timing. ⚠️ **How the load ENDS depends on where the payload came from**, and `_bufferLoadFinishOpts` (app.js) is the one place that decides it for all four fetch-and-write paths. A payload built from the server's accumulated byte history is current up to the response, so the events queued during the load already appear in it and stay DISCARDED; replaying them would duplicate output, most visibly Ink's cursor-up redraws. A pane capture (`mux-visible` or `mux-full-history`) is current only up to CAPTURE time, so `_finishBufferLoad` replays the queue from the response's own arrival timestamp (`since`) and the pre-capture events stay dropped. ⚠️ **A path that then restores a scroll position must re-take the sticky-scroll baseline** (`_syncStickyScrollBaseline`): the replay runs inside `chunkedTerminalWrite` before its promise resolves, with the terminal freshly reset, so `batchTerminalWrite` samples `_wasAtBottomBeforeWrite` as true and the next `flushPendingWrites` would scroll to the bottom over the restore. ⚠️ The cutoff is a client-side timestamp and the server broadcasts on a batch timer (8ms WebSocket, 16-50ms SSE), so a batch pending when the capture ran arrives after the response and replays although the capture holds it — bounded by one batch interval, and closable only server side by flushing that batch before the capture. Tests for the three: `test/terminal-flush-budget.test.ts` pins which sources flush, `test/terminal-buffer-flush.test.ts` pins the `since` cutoff and the baseline re-take, and `test/capture-load-window.browser.test.ts` drives both against a live server. Live output is separately one-chunk-in-flight: xterm's callback releases each 32/64 KiB write before the next is submitted, keeping the remainder in the app queue where the 128 KiB cap can observe it instead of hiding an unbounded backlog in xterm's private WriteBuffer. While WebSocket owns terminal I/O, parallel SSE terminal/output-recovery events are discarded before JSON parsing; fallback recovery is single-flight per active session so backpressure cannot start overlapping reset+replay cycles. The route exposes capture/prepare totals in `Server-Timing`, while `[TERMINAL-PERF]` separates TTFB, body/JSON, reset+parse and total time for both selection and on-demand full pulls; parse completion is not a browser compositor/GPU paint measurement. The re-pull exists because xterm's buffer is only a WINDOW onto tmux's history and two things shrink it: tmux coalesces bursty output into pane REPAINTS that overwrite rows instead of emitting linefeeds (measured: a 60-line burst added 1 row of browser scrollback and destroyed 34), and a tab switch replays only the visible frame. tmux's own history is intact throughout —Line truncated
+**Full-scrollback replay** (COD-164/#148, reworked for #205): `GET /api/sessions/:id/terminal?full=1` returns the ENTIRE tmux scrollback (capture-pane `-e -S -<lines>` bounded by the configured history limit, explicit `maxBuffer` from the terminal-history config, early byte-cap before normalization, CRLF-normalized for shell panes). On success the capture is returned ALONE (`source='mux-full-history'` — it supersedes the byte buffer; no duplication). The first load of each non-shell TUI session per page requests `full=1` (`_fullHistoryLoaded` Set in app.js — the old one-shot `_initialFullBufferLoad` flag was consumed by whichever tab auto-selected, leaving every other TUI tab one frame of history). Shell sessions instead load a bounded 1 MiB `?tail=` window on every selection and automatic drop recovery: a 100k-line shell capture can be tens of MiB, and automatically parsing it makes tab-switch latency scale with the entire session. Shell FULL history is explicit-button-only; reaching the top during an ordinary wheel/touch gesture must not reset xterm and replay the multi-megabyte capture on its main thread. That gesture instead pulls a BOUNDED window of it (`?full=1&tail=<TERMINAL_TAIL_SIZE>`, so a capture beyond 1 MiB comes back `truncationReason: 'tail'` and the banner offers the rest), and skips the replay when the window holds no more rows than the browser already has. ⚠️ **That skip runs BEFORE the downgrade guard, and it never touches the banner state**: the guard reads "smaller than the browser" as "tmux has nothing more to give", true of an unbounded capture and false of a window cut at the tail size, so a bounded window that reached it marked the session `exhausted` and removed **Load full history** while tmux still held the rest; and re-labelling the banner from a skipped payload would call a terminal that holds all of a Load full history pull "the most recent 1 MiB". A skipped window that came back truncated puts the session on the 60 s cooldown (`_fullHistoryRepullUseless`), because it can never reach anything older than the browser shows and `tail` is applied AFTER the server's synchronous `capture-pane` of the whole history, so each ask costs every client an event-loop stall (about 0.3 s at 100k lines); an untruncated one keeps the 4 s cooldown, since it is all of tmux's history and the next burst can add to it. Declining the gesture outright was a dead end: after any burst a shell pane held about one screen of browser scrollback, and the button renders only once a replay was truncated, so a young shell tab had no way back to history tmux still held. Other modes may still re-pull `full=1` at the TOP, and pressing **Load full history** forces the request for any recoverably truncated session (`_maybeRefetchFullHistory`, 4s per-session gesture cooldown, in-flight + tab-switch guards, viewport position held across the replay); Shell full pulls are not retained in the tab cache, so the next switch stays bounded. Chunked replay enqueues 32 KiB pieces across safe yields, appends an xterm parse marker, then releases the live-output gate; output arriving after that release stays ordered behind the snapshot, and the marker callback supplies accurate parse timing. ⚠️ **How the load ENDS depends on where the payload came from**, and `_bufferLoadFinishOpts` (app.js) is the one place that decides it for all four fetch-and-write paths. A payload built from the server's accumulated byte history is current up to the response, so the events queued during the load already appear in it and stay DISCARDED; replaying them would duplicate output, most visibly Ink's cursor-up redraws. A pane capture (`mux-visible` or `mux-full-history`) is current only up to CAPTURE time, so `_finishBufferLoad` replays the queue from the response's own arrival timestamp (`since`) and the pre-capture events stay dropped. ⚠️ **A path that then restores a scroll position must re-take the sticky-scroll baseline** (`_syncStickyScrollBaseline`): the replay runs inside `chunkedTerminalWrite` before its promise resolves, with the terminal freshly reset, so `batchTerminalWrite` samples `_wasAtBottomBeforeWrite` as true and the next `flushPendingWrites` would scroll to the bottom over the restore. ⚠️ The cutoff is a client-side timestamp and the server broadcasts on a batch timer (8ms WebSocket, 16-50ms SSE), so a batch pending when the capture ran arrives after the response and replays although the capture holds it — bounded by one batch interval, and closable only server side by flushing that batch before the capture. Tests for the three: `test/terminal-flush-budget.test.ts` pins which sources flush, `test/terminal-buffer-flush.test.ts` pins the `since` cutoff and the baseline re-take, and `test/capture-load-window.browser.test.ts` drives both against a live server. Live output is separately one-chunk-in-flight: xterm's callback releases each 32/64 KiB write before the next is submitted, keeping the remainder in the Line truncated
 
 **A capture reports the geometry it was taken at** (#435): a visible frame repaints each row at an absolute position, counting up to the pane's height and out to the pane's width, so a terminal smaller than that pane damages it two ways at once. Too short and every address past the browser's own height clamps onto the last line, overwriting the rows underneath (measured: against a 50-row pane, a 30-row terminal rendered 28 of a 45-line command and drew the survivors twice). Too narrow and each row is painted out to the pane's width, so the browser wraps every painted row and the wrap on the last one scrolls the whole frame up by one. Nothing in the response used to say what geometry the frame was built for, so the client could not see either case. `PaneCaptureOptions.capturedGeometry` carries it out, and the terminal response publishes it as `captureCols`/`captureRows`. ⚠️ **Both fields are ABSENT unless a frame was really positioned**, and every consumer must test `Number.isFinite` rather than truthiness: `mux-visible` is necessary but not sufficient, because when the `display-message` cursor query fails `capturePaneBuffer` skips the snapshot repaint and returns the raw capture, and the route still labels that non-empty body `mux-visible`. A body that positioned nothing has no geometry to describe and nothing to repair, so a comparison that fires there buys a second capture, a reset plus chunked rewrite, a dropped and reopened WebSocket and a discarded xterm snapshot for no gain. ⚠️ **The comparison runs on `mux-visible` ONLY.** A full-history body is linear scrollback closed by a RELATIVE cursor move, which is relative precisely so the browser's row count need not match the pane's, and a byte-history body carries no row alignment at all, so a size mismatch damages neither and a replay repairs neither. That gate matters because the first select of every non-shell session per page takes the full-history path, where an ungated comparison would fire most often on the one response it cannot help, at the price of a second whole-scrollback capture. ⚠️ **The replay is capped at one attempt and latches per session when it cannot converge.** `resizeRetry` stops two competing fits trading replays forever; a pane already drawing at the size just requested is left alone, because a retry would capture the identical frame; and a pass that still does not converge joins `_geometryRetryUseless`, so the case `Session.resize` declines outright (a small viewport while a desktop viewport's size claim is live, where the retry re-sends the same declined resize and captures the same pane) costs one attempt per session per page load instead of one per select. ⚠️ **The clamp used to manufacture that equality, and no longer can** (#464). This paragraph previously explained it as the signature of a clamp: `getTerminalDimensions()` floors at 40x10 while `fitAddon.fit()` did not, so a terminal under 40 columns or 10 rows reported a pane permanently bigger than itself and would replay on every tab switch. That divergence is fixed at the source — `syncTerminalGeometry()` (terminal-ui.js) fits, floors and APPLIES in one step, so the browser terminal IS the size it reports. The only remaining reason the two can differ is a resize the server declined, which the server now reports back (`Session.ptyGeometry`, the `{"t":"zc"}` frame and the resize response) for the client to adopt by COLUMNS. The equality guard stays, for the plain case of a pane already at the requested size. ⚠️ **A retry pass must not re-arm `_fullHistoryLoaded`**: it did not consume the full-history pull, and re-arming it would spend a whole-scrollback capture on the next select. That branch is currently unreachable by construction, since reaching it needs `source === 'mux-visible'` while a `full=1` pass is answered `mux-full-history` or `history`; a static test over the source is the habit this repo uses for an invariant nothing can execute. Tests: `test/capture-geometry-retry.browser.test.ts` (eight cases, five of which fail against the merge base), `test/tmux-capture-full-history.test.ts`, `test/routes/session-routes.test.ts`.