Merge pull request #470 from rounakdatta/fix/dropped-output-recovery

fix(terminal): recover a dropped output frame, do not merely schedule it

CLAUDE.md

@@ -261,7 +261,7 @@ Codeman is a Claude Code session manager with web interface and autonomous Ralph
 
 **Circuit breakers**: the Ralph breaker prevents respawn thrashing (`CLOSED` → `HALF_OPEN` → `OPEN`; reset via `/api/sessions/:id/ralph-circuit-breaker/reset`). **Distinct: the PTY-exit breaker** (`session-pty-exit-breaker.ts`) trips after repeated rapid PTY exits and blocks auto-restarts. ⚠️ It resets ONLY via an explicit `{clearBreaker:true}` body on `POST /api/sessions/:id/interactive`; the frontend's auto-reattach in `selectSession()` sends no body and must never clear it. → [architecture-invariants#circuit-breakers-ralph--pty-exit](docs/architecture-invariants.md#circuit-breakers-ralph-and-pty-exit)
 
-**Full-scrollback replay**: `GET /api/sessions/:id/terminal?full=1` returns the entire tmux scrollback, bounded by the configured history limit. On success the capture is returned ALONE (`source='mux-full-history'`), superseding the byte buffer so nothing duplicates. The first load of each non-shell TUI session per page requests `full=1` (`_fullHistoryLoaded` Set); Shell selection and automatic drop recovery always use a bounded 1 MiB `?tail=` window. Shell loads the rest only when **Load full history** is pressed; ordinary scrolling must not trigger a multi-megabyte reset+replay on xterm's main thread. Other modes may re-pull at the TOP (cooldown-guarded — tmux repaints bursty output in place, so browser scrollback shrinks while tmux's history stays complete). Live writes are one-chunk-in-flight, released by xterm's parse callback, so xterm's private queue cannot bypass the browser's 128 KiB render cap. While WebSocket owns terminal I/O, duplicate SSE terminal events are dropped before JSON parsing, and recovery is single-flight per active session. ⚠️ **A `full=1` capture ENDS with a cursor move back to the pane's own caret position**, counted UP from the last replayed row — without it the caret stays where the last character landed, which for an agent CLI is the status line, and every cursor-relative update the CLI sends afterwards is measured from the wrong row. The move is relative, not `CUP`: absolute row addressing is only right while the browser's rows equal the pane's, and `resizeWindow` does not wait for tmux, so a capture can be taken before a requested resize applies. That makes row alignment load-bearing on this path: no transform that can DELETE A LINE may run over the capture, so it keeps its trailing blank rows and skips redraw-bloat stripping, the banner trim and the leading-whitespace strip. ⚠️ Those three skips key on whether a capture actually CAME BACK (`isFullCapture`), never on `?full=1` alone — the fallback to the byte history is a stream of successive frames that must still be stripped, and a session with no mux takes it on every load. A capture holding nothing visible returns '' so the byte history survives instead of a blank screen replacing it. ⚠️ A full re-pull must never DOWNGRADE the buffer: a repaint-mode CLI pane keeps no tmux history, so its capture is one frame and the reset+rewrite would delete history mid-scroll — `_replayWouldShrinkBuffer()` refuses it and slows that session's cooldown to 60s. ⚠️ **A visible capture now REPORTS the geometry it was taken at** (`captureCols`/`captureRows`, #435), because a frame built for a pane taller or wider than the browser is damaged two ways at once (overflow rows clamp onto the last line; a narrower browser wraps every painted row) and nothing in the response used to say so. Both fields are ABSENT when no frame was positioned, so every consumer tests `Number.isFinite`, never truthiness: a `display-message` cursor query that fails makes `capturePaneBuffer` return the raw capture while the route still labels it `mux-visible`. The comparison runs on `mux-visible` ONLY, the replay is capped at one attempt, and a pane that cannot be sized to fit latches in `_geometryRetryUseless` so it is diagnosed once per session rather than on every tab switch. → [architecture-invariants#full-scrollback-replay](docs/architecture-invariants.md#full-scrollback-replay)
+**Full-scrollback replay**: `GET /api/sessions/:id/terminal?full=1` returns the entire tmux scrollback, bounded by the configured history limit. On success the capture is returned ALONE (`source='mux-full-history'`), superseding the byte buffer so nothing duplicates. The first load of each non-shell TUI session per page requests `full=1` (`_fullHistoryLoaded` Set); Shell selection and automatic drop recovery always use a bounded 1 MiB `?tail=` window. Shell loads the rest only when **Load full history** is pressed; ordinary scrolling must not trigger a multi-megabyte reset+replay on xterm's main thread. Other modes may re-pull at the TOP (cooldown-guarded — tmux repaints bursty output in place, so browser scrollback shrinks while tmux's history stays complete). Live writes are one-chunk-in-flight, released by xterm's parse callback, so xterm's private queue cannot bypass the browser's 128 KiB render cap. ⚠️ **A frame dropped at that cap MUST be recovered, and the recovery must verify itself** (`_scheduleDroppedOutputRecovery`, app.js): a hole in a TUI byte stream is a desynced cursor, which is muffled text (#464). It was a fire-and-forget 2s timer that nulled its own handle and then called `_onSessionNeedsRefresh()` — whose early returns (a buffer load in flight, a refresh already owning the session) are MOST likely to be true during exactly the burst that caused the drop, so the recovery was lost silently and the bytes were never replayed. `_onSessionNeedsRefresh` now returns whether it actually repainted, and the scheduler re-arms while it has not, bounded by `DROP_RECOVERY_MAX_ATTEMPTS` because every reason it can be skipped is transient contention. The same debounce still collapses a burst into one attempt. Tests: `test/dropped-output-recovery.test.ts`, whose retry case and no-retry case only pin the fix as a pair. While WebSocket owns terminal I/O, duplicate SSE terminal events are dropped before JSON parsing, and recovery is single-flight per active session. ⚠️ **A `full=1` capture ENDS with a cursor move back to the pane's own caret position**, counted UP from the last replayed row — without it the caret stays where the last character landed, which for an agent CLI is the status line, and every cursor-relative update the CLI sends afterwards is measured from the wrong row. The move is relative, not `CUP`: absolute row addressing is only right while the browser's rows equal the pane's, and `resizeWindow` does not wait for tmux, so a capture can be taken before a requested resize applies. That makes row alignment load-bearing on this path: no transform that can DELETE A LINE may run over the capture, so it keeps its trailing blank rows and skips redraw-bloat stripping, the banner trim and the leading-whitespace strip. ⚠️ Those three skips key on whether a capture actually CAME BACK (`isFullCapture`), never on `?full=1` alone — the fallback to the byte history is a stream of successive frames that must still be stripped, and a session with no mux takes it on every load. A capture holding nothing visible returns '' so the byte history survives instead of a blank screen replacing it. ⚠️ A full re-pull must never DOWNGRADE the buffer: a repaint-mode CLI pane keeps no tmux history, so its capture is one frame and the reset+rewrite would delete history mid-scroll — `_replayWouldShrinkBuffer()` refuses it and slows that session's cooldown to 60s. ⚠️ **A visible capture now REPORTS the geometry it was taken at** (`captureCols`/`captureRows`, #435), because a frame built for a pane taller or wider than the browser is damaged two ways at once (overflow rows clamp onto the last line; a narrower browser wraps every painted row) and nothing in the response used to say so. Both fields are ABSENT when no frame was positioned, so every consumer tests `Number.isFinite`, never truthiness: a `display-message` cursor query that fails makes `capturePaneBuffer` return the raw capture while the route still labels it `mux-visible`. The comparison runs on `mux-visible` ONLY, the replay is capped at one attempt, and a pane that cannot be sized to fit latches in `_geometryRetryUseless` so it is diagnosed once per session rather than on every tab switch. → [architecture-invariants#full-scrollback-replay](docs/architecture-invariants.md#full-scrollback-replay)
 
 **Split-pane sessions** (`showSplitButton`, header button, default OFF, desktop-only, per-device): a second live session ("Pane B") beside the active one, in its own `SplitTerminalPane` (terminal-split.js) with its own xterm + WebSocket, resizable via a draggable divider. Deliberately plainer than the primary pane — no local-echo overlay, CJK IME, or touch handlers — and NOT persisted across reloads. → [architecture-invariants#split-pane-sessions](docs/architecture-invariants.md#split-pane-sessions)