fix(terminal): let a Shell pane's scroll-up reach tmux history

A burst of output leaves a Shell pane with about one screen of browser
scrollback, because tmux repaints the burst instead of scrolling it,
while tmux itself keeps every line. Shell declined the scroll-to-top
re-pull other modes use, and the Load full history button renders only
once a replay was truncated, so a Shell tab under 1 MiB could not
scroll back at all.

The scroll gesture now pulls ?full=1&tail=TERMINAL_TAIL_SIZE, the same
bound a tab switch loads; the route's existing tail cut marks longer
histories 'tail', so the banner still offers the unbounded pull. A
window no longer than the browser's buffer is not rewritten.

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

docs/wiki/The-Dashboard.md

@@ -151,8 +151,9 @@ Worth knowing:
 
 - **Scrollback.** Agent/TUI sessions pull their entire tmux scrollback on first open.
   Shell sessions open from a bounded recent tail so a large transcript cannot stall tab
-  switching; press **Load full history** to pull the rest explicitly. Ordinary Shell scrolling
+  switching. Scrolling to the top of a Shell pane pulls the most recent 1 MiB of its tmux
-  and automatic output recovery stay within the bounded browser buffer.
+  history; press **Load full history** to pull the rest explicitly. Automatic output
+  recovery stays within the bounded browser buffer.
 - **Wheel and touch scrolling** are forwarded into Claude's own transcript on recent Claude
   versions, so the wheel scrolls the conversation rather than the terminal. `Shift+Wheel` is
   always local scrollback. Other CLIs scroll locally.

src/web/public/app.js

@@ -6320,10 +6320,15 @@ class CodemanApp {
     if (!sessionId || this._fullHistoryRepullInFlight || this._isLoadingBuffer) return;
     if (this.detachedSessions?.has(sessionId)) return;
     const session = this.sessions.get(sessionId);
-    // A shell's full capture can be many megabytes. Replaying it from an
+    // A shell's full capture can be many megabytes, and replaying all of it from
-    // ordinary scroll gesture blocks xterm's main thread, so keep that cost
+    // an ordinary scroll gesture blocks xterm's main thread. So a shell scroll
-    // behind the explicit "Load full history" button.
+    // pulls a BOUNDED window of tmux's full history (the same 1 MiB a tab switch
-    if (!force && session?.mode === 'shell') return;
+    // loads, but of the scrollback rather than the visible frame) and the
+    // unbounded pull stays behind the "Load full history" button. Declining
+    // outright left a shell pane about one screen of browser scrollback after any
+    // burst, and the button only renders once a replay was truncated, so a young
+    // shell tab had no way back to output tmux was still holding.
+    const boundedShellPull = !force && session?.mode === 'shell';
     const now = Date.now();
     // Momentum scrolling fires this dozens of times per flick, and a burst of new
     // output is the normal reason to want a re-pull, so cooldown rather than latch.
@@ -6336,7 +6341,12 @@ class CodemanApp {
     this._fullHistoryRepullInFlight = true;
     try {
       const requestStartedAt = performance.now();
-      const capture = await this._fetchTerminalCapture(`/api/sessions/${sessionId}/terminal?full=1`, { full: true });
+      const capture = await this._fetchTerminalCapture(
+        boundedShellPull
+          ? `/api/sessions/${sessionId}/terminal?full=1&tail=${TERMINAL_TAIL_SIZE}`
+          : `/api/sessions/${sessionId}/terminal?full=1`,
+        { full: true }
+      );
       const headersReceivedAt = capture.headersAt;
       const payload = capture.json?.data ?? {};
       const bodyParsedAt = performance.now();
@@ -6368,6 +6378,20 @@ class CodemanApp {
         this._setHistoryTruncation(sessionId, { ...payload, exhausted: true });
         return;
       }
+      // A bounded window no longer than the browser's buffer buys nothing, and
+      // resetting to rewrite it would jump the viewport on every scroll that
+      // outlasts the cooldown at the top. An untruncated window IS all of tmux's
+      // history, so nothing is missing; a window cut at the tail size would trade
+      // more old rows than it recovers, and its 'tail' truncation keeps the
+      // banner offering the unbounded pull. Not latched as useless: the next
+      // burst of output can put more history in tmux than the browser has.
+      if (
+        boundedShellPull &&
+        this._estimateReplayRows(buffer, this.terminal.cols) <= this.terminal.buffer.active.length
+      ) {
+        this._setHistoryTruncation(sessionId, payload);
+        return;
+      }
       this._setHistoryTruncation(sessionId, payload);
       this._fullHistoryRepullUseless?.delete(sessionId);
       const rowsBefore = this.terminal.buffer.active.length;

src/web/public/terminal-ui.js

@@ -3304,9 +3304,9 @@ Object.assign(CodemanApp.prototype, {
   /**
    * Post-scroll companion to _noteTerminalUserScroll: hitting the TOP of the
    * buffer while scrolling up gives the app a chance to pull the rest of tmux's
-   * scrollback (issue #205, see _maybeRefetchFullHistory). Shell sessions decline
+   * scrollback (issue #205, see _maybeRefetchFullHistory). Shell sessions pull a
-   * automatic pulls because their captures can be large; their banner button is
+   * bounded window because their captures can be large; their banner button is
-   * the explicit path. Must be called AFTER scrollLines(), since the check is on
+   * the unbounded path. Must be called AFTER scrollLines(), since the check is on
    * the resulting position, and it is deliberately not folded into
    * _noteTerminalUserScroll for exactly that reason.
    */

test/history-truncation-notice.test.ts

@@ -122,7 +122,7 @@ describe('the in-terminal truncation line is gone (static guard)', () => {
     expect(app).not.toContain('earlier output truncated for performance');
   });
 
-  it('loads a bounded shell tail first and keeps full history user-triggered', () => {
+  it('loads a bounded shell tail first and keeps unbounded full history user-triggered', () => {
     const app = readFileSync(resolve(PUBLIC, 'app.js'), 'utf8');
     expect(app).toContain("session?.mode !== 'shell' && !this._fullHistoryLoaded.has(sessionId)");
     expect(app).toContain("!restoredSnapshot && session?.mode !== 'shell'");
@@ -131,10 +131,13 @@ describe('the in-terminal truncation line is gone (static guard)', () => {
     // an abort deadline (a `?full=1` body can be megabytes and used to hang
     // indefinitely on a stalled mobile link). The URL and the full-vs-tail
     // decision this guard exists to pin are unchanged.
-    expect(app).toContain('this._fetchTerminalCapture(`/api/sessions/${sessionId}/terminal?full=1`, { full: true })');
+    expect(app).toContain(': `/api/sessions/${sessionId}/terminal?full=1`,\n        { full: true }');
     expect(app).toContain("if (this.sessions.get(sessionId)?.mode !== 'shell')");
     expect(app).toContain("if (session?.mode === 'shell')");
-    expect(app).toContain("if (!force && session?.mode === 'shell') return;");
+    // A shell scroll gesture pulls a BOUNDED window of full history; only the
+    // button pulls all of it (behaviour pinned in shell-scroll-history-pull.test.ts).
+    expect(app).toContain("const boundedShellPull = !force && session?.mode === 'shell';");
+    expect(app).toContain('`/api/sessions/${sessionId}/terminal?full=1&tail=${TERMINAL_TAIL_SIZE}`');
     expect(app).toContain("trigger: force ? 'full-history-button' : 'full-history-scroll'");
   });
 

test/shell-scroll-history-pull.test.ts

@@ -0,0 +1,142 @@
+/**
+ * @fileoverview A shell pane's scroll-up must reach the history tmux still holds.
+ *
+ * tmux repaints a burst of output instead of scrolling it, so after `cat` of a
+ * file longer than the screen the browser holds about one screen of scrollback
+ * while tmux holds all of it. Other modes recover it by re-pulling `?full=1`
+ * when the wheel reaches the top (`_maybeRefetchFullHistory`, issue #205). Shell
+ * declined that gesture outright to keep a multi-megabyte capture off xterm's
+ * main thread, leaving only the "Load full history" button, and that button
+ * renders only once a replay was truncated. A young shell tab therefore had no
+ * way to scroll back at all.
+ *
+ * The gesture now pulls a BOUNDED window (`?full=1&tail=TERMINAL_TAIL_SIZE`),
+ * the button stays the unbounded path, and a window the browser already holds
+ * in full is not rewritten.
+ *
+ * The method is extracted from app.js and run in a `vm` against stubs (no jsdom
+ * on this box; see connection-indicator.test.ts), with the REAL row estimators
+ * from terminal-ui.js, which decide both the downgrade and the no-gain skip.
+ */
+import { readFileSync } from 'node:fs';
+import { performance } from 'node:perf_hooks';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it, vi } from 'vitest';
+
+const PUBLIC = resolve(import.meta.dirname, '../src/web/public');
+const TERMINAL_TAIL_SIZE = 1024 * 1024;
+
+function methodSource(source: string, method: string): string {
+  const start = source.search(new RegExp(`^ {2}(?:async )?${method}\\(`, 'm'));
+  expect(start, `${method} not found`).toBeGreaterThan(-1);
+  const next = /^ {2}(?:async )?[A-Za-z_$][\w$]*\(/m.exec(source.slice(start + 1));
+  return next ? source.slice(start, start + 1 + next.index) : source.slice(start);
+}
+
+/** Real terminal-ui.js mixin, for `_estimateReplayRows` / `_replayWouldShrinkBuffer`. */
+function loadTerminalMixin(): Record<string, unknown> {
+  const source = readFileSync(resolve(PUBLIC, 'terminal-ui.js'), 'utf8');
+  const FakeCodemanApp = function () {} as unknown as { prototype: Record<string, unknown> };
+  const context = vm.createContext({
+    console,
+    performance,
+    setTimeout,
+    clearTimeout,
+    setInterval: vi.fn(),
+    clearInterval: vi.fn(),
+    requestAnimationFrame: vi.fn(),
+    CodemanApp: FakeCodemanApp,
+    window: { addEventListener: vi.fn(), removeEventListener: vi.fn() },
+    document: { addEventListener: vi.fn() },
+  });
+  vm.runInContext(source, context);
+  return FakeCodemanApp.prototype;
+}
+
+function loadRefetch(): (this: unknown, opts?: { force?: boolean }) => Promise<void> {
+  const app = readFileSync(resolve(PUBLIC, 'app.js'), 'utf8');
+  const body = methodSource(app, '_maybeRefetchFullHistory');
+  const context = vm.createContext({ performance, TERMINAL_TAIL_SIZE, TERMINAL_CHUNK_SIZE: 32 * 1024 });
+  return vm.runInContext(`({ ${body} })._maybeRefetchFullHistory`, context);
+}
+
+const mixin = loadTerminalMixin();
+const refetch = loadRefetch();
+const lines = (n: number) => Array.from({ length: n }, (_, i) => `line ${i}`).join('\r\n');
+
+function makeApp(mode: string, { bufferRows, capture }: { bufferRows: number; capture: string }) {
+  const urls: string[] = [];
+  const app = {
+    activeSessionId: 's1',
+    sessions: new Map([['s1', { mode }]]),
+    detachedSessions: new Set<string>(),
+    _fullHistoryRepullInFlight: false,
+    _isLoadingBuffer: false,
+    _fullHistoryRepullAt: new Map<string, number>(),
+    _fullHistoryRepullUseless: new Set<string>(),
+    terminalBufferCache: new Map<string, string>(),
+    terminal: {
+      cols: 80,
+      rows: 30,
+      buffer: { active: { length: bufferRows } },
+      scrollToLine: vi.fn(),
+      scrollToTop: vi.fn(),
+    },
+    _estimateReplayRows: mixin._estimateReplayRows,
+    _replayWouldShrinkBuffer: mixin._replayWouldShrinkBuffer,
+    _fetchTerminalCapture: vi.fn(async (url: string) => {
+      urls.push(url);
+      return {
+        headersAt: performance.now(),
+        headers: { get: () => '' },
+        json: { data: { terminalBuffer: capture, source: 'mux-full-history' } },
+      };
+    }),
+    _recordTerminalLoadTiming: vi.fn(),
+    _logScrollRouting: vi.fn(),
+    _setHistoryTruncation: vi.fn(),
+    _resetTerminalForReplay: vi.fn(),
+    _bufferLoadFinishOpts: vi.fn(() => ({})),
+    chunkedTerminalWrite: vi.fn(async () => ({ parsedAt: performance.now(), bufferLength: 400, completed: true })),
+    _syncStickyScrollBaseline: vi.fn(),
+  };
+  return { app, urls };
+}
+
+describe('shell scroll-up pulls a bounded window of tmux history', () => {
+  it('a shell scroll gesture requests full history bounded by the tail size', async () => {
+    const { app, urls } = makeApp('shell', { bufferRows: 40, capture: lines(300) });
+    await refetch.call(app);
+    expect(urls).toEqual([`/api/sessions/s1/terminal?full=1&tail=${TERMINAL_TAIL_SIZE}`]);
+    // It then actually replays the recovered history.
+    expect(app._resetTerminalForReplay).toHaveBeenCalledTimes(1);
+    expect(app.chunkedTerminalWrite).toHaveBeenCalledTimes(1);
+  });
+
+  it('the Load full history button stays unbounded for a shell', async () => {
+    const { app, urls } = makeApp('shell', { bufferRows: 40, capture: lines(300) });
+    await refetch.call(app, { force: true });
+    expect(urls).toEqual(['/api/sessions/s1/terminal?full=1']);
+  });
+
+  it('other modes keep the unbounded scroll pull', async () => {
+    const { app, urls } = makeApp('claude', { bufferRows: 40, capture: lines(300) });
+    await refetch.call(app);
+    expect(urls).toEqual(['/api/sessions/s1/terminal?full=1']);
+  });
+
+  it('a bounded window the browser already holds is not rewritten', async () => {
+    // Browser already has every row the window carries: resetting to rewrite
+    // it would jump the viewport on every scroll that outlasts the cooldown.
+    const { app } = makeApp('shell', { bufferRows: 320, capture: lines(300) });
+    await refetch.call(app);
+    expect(app._resetTerminalForReplay).not.toHaveBeenCalled();
+    expect(app.chunkedTerminalWrite).not.toHaveBeenCalled();
+    // Not latched as useless: more output can put more history in tmux.
+    expect(app._fullHistoryRepullUseless.has('s1')).toBe(false);
+    // The truncation state is still recorded, so a window capped at the tail
+    // size keeps offering the button.
+    expect(app._setHistoryTruncation).toHaveBeenCalledTimes(1);
+  });
+});