perf(terminal): tab switch to a pane with no scrollback loads only its screen

After a session's first load per page, a tab switch fetched a 1 MiB tail of
the server's byte stream. For a fullscreen claude pane (alternate screen,
tmux `history_size` 0) that tail is old repaints of one frame, and an idle
tab parsed it twice (the cached copy, then the fresh one).

Every terminal response now records `paneHistoryLines` per session, and
`selectSession` takes `full=1` whenever the last report was 0. For such a
pane that capture is the visible frame, a few KB, which is also what a page
load already shows. A response without the field forgets the entry, so
unknown never counts as empty; a pane that starts keeping history goes back
to the tail on the next switch. Shells and the refresh path are unchanged.

Measured with real prod payloads through the real client in chromium:
median switch 1.26-1.66 s -> 0.10-0.19 s, parse ~1.1-1.5 s -> 2-3 ms.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -767,6 +767,12 @@ class CodemanApp {
     // their scrollback can be very large; full history stays available on demand.
     // Tracked PER SESSION rather than as a single "first load" flag (issue #205).
     this._fullHistoryLoaded = new Set();
+    // Rows of scrollback tmux reported above the visible frame on this session's
+    // last capture (`paneHistoryLines`). 0 marks a pane with no history to load
+    // (a fullscreen CLI in the alternate screen), which a tab switch serves from
+    // the small `full=1` capture instead of the 1 MiB byte tail: see
+    // _notePaneHistory and selectSession.
+    this._paneHistoryLines = new Map(); // Map<sessionId, number>
     // Cooldown per session for the scroll-to-top "load more history" re-pull.
     this._fullHistoryRepullAt = new Map(); // Map<sessionId, timestamp>
     this._fullHistoryRepullInFlight = false;
@@ -3193,6 +3199,7 @@ class CodemanApp {
         headersReceivedAt = capture.headersAt;
         data = capture.json?.data ?? {};
       }
+      this._notePaneHistory?.(sessionId, data);
       // Bail on a tab switch mid-fetch: writing here would paint this session's
       // history into the terminal the user is now looking at. The window is two
       // fetches wide in the fallback case, so this guard is not optional.
@@ -8552,6 +8559,7 @@ class CodemanApp {
       const headersReceivedAt = capture.headersAt;
       const payload = capture.json?.data ?? {};
       const bodyParsedAt = performance.now();
+      this._notePaneHistory?.(sessionId, payload);
       const buffer = payload.terminalBuffer;
       const timing = {
         trigger: force ? 'full-history-button' : 'full-history-scroll',
@@ -8700,9 +8708,27 @@ class CodemanApp {
   _clearHistoryTruncation(sessionId) {
     this._historyTruncation?.delete(sessionId);
     this._historyNoticeDismissed?.delete(sessionId);
+    this._paneHistoryLines?.delete(sessionId);
     if (sessionId === this.activeSessionId) this._renderHistoryTruncationBanner();
   }
 
+  /**
+   * Remember how much scrollback tmux holds for a session's pane, from any
+   * terminal response. Called whether or not the response was written, since
+   * it describes the PANE, not the payload. A response without the field (a
+   * byte-history fallback, an older server) forgets it, so the next tab switch
+   * goes back to the bounded tail: unknown never counts as empty.
+   */
+  _notePaneHistory(sessionId, payload) {
+    if (!sessionId) return;
+    const lines = payload?.paneHistoryLines;
+    if (typeof lines === 'number' && Number.isFinite(lines) && lines >= 0) {
+      (this._paneHistoryLines ||= new Map()).set(sessionId, lines);
+    } else {
+      this._paneHistoryLines?.delete(sessionId);
+    }
+  }
+
   /**
    * Bring the partial-history notice up for `sessionId`, or retire it (null).
    *
@@ -9166,7 +9192,23 @@ class CodemanApp {
       // automatically replaying all of them makes tab selection scale with the
       // entire session. Load its bounded 1MB tail first; the existing truncation
       // banner action fetches ?full=1 when the user explicitly asks for it.
-      const useFullHistory = session?.mode !== 'shell' && !this._fullHistoryLoaded.has(sessionId);
+      //
+      // A TUI pane whose last capture reported NO tmux scrollback (fullscreen
+      // claude: it lives in the alternate screen and keeps its transcript itself)
+      // takes `full=1` on every switch, not only the first. For such a pane that
+      // capture IS the visible frame, a few KB, while the tail is 1 MiB of the
+      // byte stream's old repaints. Measured on 11 live fullscreen claude panes:
+      // the tail took 250-1070 ms on the server and 70-510 ms to parse, and
+      // rendered one frame repeated (450 rows, 44 distinct); `full=1` took
+      // 150-330 ms and returned 0.9-5.6 KB. It also matches what a page load
+      // already shows, and an empty local buffer is what lets
+      // _maybePageCliTranscript send a wheel to the CLI's own transcript. The
+      // response re-reports the count, so a pane that starts keeping history
+      // (claude switched to its inline view) goes back to the tail on the next
+      // switch.
+      const paneKeepsNoHistory = this._paneHistoryLines?.get(sessionId) === 0;
+      const useFullHistory =
+        session?.mode !== 'shell' && (paneKeepsNoHistory || !this._fullHistoryLoaded.has(sessionId));
       if (useFullHistory) this._fullHistoryLoaded.add(sessionId);
       const fetchStartedAt = performance.now();
       const tailUrl = `/api/sessions/${sessionId}/terminal?tail=${TERMINAL_TAIL_SIZE}`;
@@ -9196,6 +9238,7 @@ class CodemanApp {
       }
       const data = capture.json?.data ?? {};
       const bodyParsedAt = performance.now();
+      this._notePaneHistory?.(sessionId, data);
       // How this load must end, decided here because `chunkedTerminalWrite` is
       // what actually ends it for a non-empty buffer. A tmux pane capture is a
       // point-in-time frame, so nothing that reached the browser after the