fix(terminal): preserve scroll intent across keyboard resize, surface history truncation

Closes #259, closes #258. Both bottom out in the same gap: nothing tracked
whether the user was following live output or reading history.

#259 — the keyboard path forced the terminal to the bottom unconditionally
(onKeyboardShow/onKeyboardHide passed scrollToBottom:true, applied with no
check), so opening the keyboard while scrolled up yanked the user down. The
settle cycle now captures intent on its FIRST event, before any fit() has
reflowed the buffer, and returns to that anchor when the user was reading.
A later capture would read an already-moved viewportY, which is why the
capture point matters. The param is renamed restoreScroll to match.

Separately, flushPendingWrites gated viewport preservation on
_hasRecentUserScrollUp(), a 1500ms decay window, so a user who scrolled up and
then actually READ for longer lost protection mid-read. Being scrolled up IS
the intent however long ago it was expressed, so it now keys off position.
The recency window stays as a race guard on the sticky scroll-to-bottom.

The full-history repull already held the user's place and is unchanged.

#258 — truncation was reported by a grey line written INTO the terminal
("earlier output truncated"), which scrolls away with the output it describes,
cannot be acted on, and said the same thing whether the rest was one click away
or gone forever. The server set one `truncated` boolean at two sites meaning
opposite things, and the client discarded fullSize and source entirely.

The route now reports truncationReason ('tail' = intentional partial replay,
the rest is retained; 'capped' = the byte ceiling dropped it) plus
retainedBytes, and 'capped' is not downgraded by a later tail cut. The client
renders a dismissible banner outside terminal output with three honest states:
recoverable (offers Load full history), at-ceiling, and exhausted. The Load
button forces past the scroll cooldown but NOT past _replayWouldShrinkBuffer,
which still refuses a downgrade for repaint-mode panes.

The banner is an overlay, not a flex child: FitAddon derives rows/cols from the
terminal parent's computed height, so occupying real layout space would SIGWINCH
the CLI on every truncation-state change.

Verified in a real browser on the 7 skins: banner text and button clear 4.5:1
contrast on all of them, and terminal height is byte-identical with the banner
shown. The first cut used --bg-elevated and --accent-muted, which do not exist,
so light skins rendered a hardcoded dark bar under dark text; it now uses only
tokens every skin redefines.

test/terminal-scroll-intent.test.ts lives outside test/mobile/ deliberately —
that suite is excluded from test:ci, so a guard placed there is invisible to CI.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -4509,28 +4509,36 @@ class CodemanApp {
    * gets a much longer cooldown so a hollow pane stops re-fetching megabytes on
    * every scroll-up (issue #205, round 2).
    */
-  async _maybeRefetchFullHistory() {
+  async _maybeRefetchFullHistory({ force = false } = {}) {
     const sessionId = this.activeSessionId;
     if (!sessionId || this._fullHistoryRepullInFlight || this._isLoadingBuffer) return;
     if (this.detachedSessions?.has(sessionId)) return;
     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.
+    // `force` is the user pressing "Load full history" (#258): they asked once,
+    // explicitly, so the scroll-gesture cooldown does not apply. The downgrade
+    // guard below still does — a forced pull must not destroy history either.
     const cooldown = this._fullHistoryRepullUseless?.has(sessionId) ? 60000 : 4000;
-    if (now - (this._fullHistoryRepullAt.get(sessionId) || 0) < cooldown) return;
+    if (!force && now - (this._fullHistoryRepullAt.get(sessionId) || 0) < cooldown) return;
     this._fullHistoryRepullAt.set(sessionId, now);
     this._fullHistoryRepullInFlight = true;
     try {
       const res = await fetch(`/api/sessions/${sessionId}/terminal?full=1`);
-      const buffer = (await res.json())?.data?.terminalBuffer;
+      const payload = (await res.json())?.data ?? {};
+      const buffer = payload.terminalBuffer;
       // Bail on a tab switch mid-fetch: writing here would paint another session's
       // history into the terminal the user is now looking at.
       if (!buffer || this.activeSessionId !== sessionId) return;
       if (this._replayWouldShrinkBuffer(buffer)) {
         (this._fullHistoryRepullUseless ||= new Set()).add(sessionId);
         this._logScrollRouting?.('repull-refused-downgrade');
+        // The browser already holds more than tmux can give back, so there is
+        // nothing further to offer and the indicator must stop promising it.
+        this._setHistoryTruncation(sessionId, { ...payload, exhausted: true });
         return;
       }
+      this._setHistoryTruncation(sessionId, payload);
       this._fullHistoryRepullUseless?.delete(sessionId);
       const rowsBefore = this.terminal.buffer.active.length;
       this._resetTerminalForReplay();
@@ -4551,6 +4559,89 @@ class CodemanApp {
     }
   }
 
+  /**
+   * Record how much history a replay actually carried, and refresh the banner.
+   *
+   * Called from every path that writes a fetched buffer into xterm. Keyed by
+   * session because the banner describes the ACTIVE tab and a background fetch
+   * must not relabel it.
+   */
+  _setHistoryTruncation(sessionId, payload = {}) {
+    if (!sessionId) return;
+    (this._historyTruncation ||= new Map()).set(sessionId, {
+      truncated: !!payload.truncated,
+      reason: payload.truncationReason ?? null,
+      source: payload.source ?? null,
+      fullSize: payload.fullSize ?? 0,
+      retainedBytes: payload.retainedBytes ?? 0,
+      // Set once a full-history pull has been refused as a downgrade: the
+      // browser holds more than the server can return, so there is no more.
+      exhausted: !!payload.exhausted,
+    });
+    if (sessionId === this.activeSessionId) this._renderHistoryTruncationBanner();
+  }
+
+  /** Drop banner state for a session that is going away. */
+  _clearHistoryTruncation(sessionId) {
+    this._historyTruncation?.delete(sessionId);
+    if (sessionId === this.activeSessionId) this._renderHistoryTruncationBanner();
+  }
+
+  /**
+   * Paint the partial-history banner for the active session.
+   *
+   * Three distinct states, because "we tailed for speed" and "the oldest output
+   * is gone forever" are not the same message and the old single boolean could
+   * not tell them apart:
+   *   - recoverable  → offer to load the rest
+   *   - exhausted    → say so plainly, offer nothing
+   *   - at the limit → the full capture ITSELF hit the byte ceiling
+   */
+  _renderHistoryTruncationBanner() {
+    const bar = document.getElementById('historyTruncationBar');
+    if (!bar) return;
+    const state = this.activeSessionId ? this._historyTruncation?.get(this.activeSessionId) : null;
+    const notice = computeHistoryTruncationNotice(state || {});
+    if (!notice.visible) {
+      bar.hidden = true;
+      return;
+    }
+
+    bar.textContent = '';
+    const label = document.createElement('span');
+    label.className = 'history-trunc-text';
+    label.textContent = notice.message;
+    bar.appendChild(label);
+
+    if (notice.canLoadMore) {
+      const btn = document.createElement('button');
+      btn.type = 'button';
+      btn.className = 'history-trunc-load';
+      btn.textContent = 'Load full history';
+      btn.onclick = () => {
+        btn.disabled = true;
+        btn.textContent = 'Loading…';
+        // Forced: the cooldown exists to throttle scroll gestures, not choices.
+        this._maybeRefetchFullHistory({ force: true }).finally(() => {
+          this._renderHistoryTruncationBanner();
+        });
+      };
+      bar.appendChild(btn);
+    }
+
+    const dismiss = document.createElement('button');
+    dismiss.type = 'button';
+    dismiss.className = 'history-trunc-dismiss';
+    dismiss.setAttribute('aria-label', 'Dismiss history notice');
+    dismiss.textContent = '×';
+    dismiss.onclick = () => {
+      bar.hidden = true;
+    };
+    bar.appendChild(dismiss);
+
+    bar.hidden = false;
+  }
+
   _shouldFocusTerminalForTabSwitch() {
     if (typeof MobileDetection === 'undefined' || !MobileDetection.isTouchDevice()) {
       return true;
@@ -4607,6 +4698,10 @@ class CodemanApp {
 
     this._cleanupPreviousSession(sessionId);
     this.activeSessionId = sessionId;
+    // Repaint the partial-history banner for the tab being switched TO. The
+    // replay paths refresh it when their fetch lands; without this the previous
+    // session's notice stays on screen until then (#258).
+    this._renderHistoryTruncationBanner();
     try { localStorage.setItem('codeman-active-session', sessionId); } catch {}
     // Narrow SSE filter to the active session — server stops streaming
     // session:terminal events for other sessions to this client. Cuts
@@ -4858,10 +4953,11 @@ class CodemanApp {
           _crashDiag.log(`REWRITE: ${(data.terminalBuffer.length/1024).toFixed(0)}KB`);
           this._setTerminalLoadState(sessionId, selectGen, 'replaying');
           this._resetTerminalForReplay();
-          // Show truncation indicator if buffer was cut
-          if (data.truncated) {
-            this.terminal.write('\x1b[90m... (earlier output truncated for performance) ...\x1b[0m\r\n\r\n');
-          }
+          // Truncation is reported OUT OF BAND (#258). This used to write a grey
+          // "... earlier output truncated ..." line into the
+          // terminal itself, which scrolls away with the output it describes,
+          // cannot be actioned, and is indistinguishable from real CLI output.
+          this._setHistoryTruncation(sessionId, data);
           // Use chunked write for large buffers to avoid UI jank
           await this.chunkedTerminalWrite(data.terminalBuffer, TERMINAL_CHUNK_SIZE, bufferLoadOwner);
           if (this._isStaleSelect(selectGen)) {
@@ -5043,6 +5139,7 @@ class CodemanApp {
     }
     this.terminalBuffers.delete(sessionId);
     this.terminalBufferCache.delete(sessionId);
+    this._clearHistoryTruncation(sessionId);
     this._xtermSnapshots?.delete(sessionId);
     try { localStorage.removeItem(`codeman-xs-${sessionId}`); } catch {}