Merge pull request #524 from timkjr/fix/split-pane-live-queue-timing

fix(split-pane): open Pane B's live queue after the response, keep the disconnected marker last

# Conflicts:
#	docs/architecture-invariants.md

docs/architecture-invariants.md

@@ -780,7 +780,7 @@ Further detail: with many sessions the horizontal strip stops being scannable, w
 
 ### Split-pane sessions
 
-**Split-pane sessions** (`showSplitButton`, header button, default OFF, per-device like `showFileViewerButton` — not in `SettingsUpdateSchema`, `displayKeys` in settings-ui.js): shows two live sessions side-by-side in one Codeman window. Pane A is the untouched, existing singleton terminal (`this.terminal`/`this._ws` in terminal-ui.js); Pane B is a new, independent `SplitTerminalPane` (terminal-split.js) with its own xterm instance and its own `/ws/sessions/:id/terminal` WebSocket. ⚠️ **Pane B is deliberately plainer than Pane A** — no local-echo overlay, no CJK IME, no touch/mobile handlers, no keyboard accessory bar — since this is a desktop-only feature (a split view needs a wide viewport) and those features exist for mobile/touch input; `.btn-split` is hard-hidden below 1180px regardless of the setting by the `@media (max-width: 1179px)` rule in styles.css (mobile.css only carries a comment pointing at it: that file loads up to 1023px, so it cannot cover the 1024-1179px tablet range the feature also needs to stay off), and the per-device setting means turning it on at a desk can never sync it onto a phone in the first place. No persistence: closing the browser tab or reloading always returns to the normal single-pane view; there is no localStorage key for split state. ⚠️ Splitting a session against itself is disallowed (the picker excludes the active session), **as is splitting against a popped-out (detached) session** — `buildSplitPickerSessions()` excludes `detachedSessions` because a detached session's own window is already claiming its PTY size, and `MAX_WS_PER_SESSION` needs no change since Pane A/B are always two different sessions. ⚠️ Either pane's session ending (deleted locally or from another client) auto-collapses the split — Pane A's session ending promotes Pane B to the new single pane via `selectSession(id, { auto: true })` (an app-driven selection, so it must not spend the session's idle alert — see the Approvals Inbox note above), never by trying to hot-swap the lightweight `SplitTerminalPane` object into the primary singleton state. ⚠️ Pane B refits on every window/sidebar/tab-rail resize via the SAME trailing-edge `ResizeObserver` callback that resizes Pane A (`throttledResize` in terminal-ui.js) — it only ever measured Pane A's own container, so without an explicit `this._splitPane?.fit()` call there Pane B silently kept its stale PTY size through every resize that did not happen to be a divider drag. ⚠️ A dropped WebSocket leaves Pane B visibly dead (a message written into its own xterm buffer) rather than silently swallowing keystrokes with nothing on screen to explain why — there is no reconnect logic for v1, matching the "deliberately plainer than Pane A" design. Related but distinct: `detachSession()` already opens one session in a separate OS-level browser window (`isSoloWindow`) — that is prior art for "two sessions visible at once" but not for one window with a draggable in-page divider, which is what this feature adds. ⚠️ Pane B installs its own `attachCustomKeyEventHandler` gating the same app-level chords Pane A's own handler gates (command palette, Alt+1-9/[/] tab nav, Alt+B sidebar toggle, Ctrl+Z suspend, Shift/Ctrl+Enter newline, smart-copy Ctrl+C) — without it the document capture-phase handler's `preventDefault()` (which never stops xterm) let each chord ALSO write its raw byte/escape sequence into Pane B's live PTY on top of whatever the app action did to Pane A (COD-153). Ctrl+Z is swallowed unless Pane B's own session is `mode === 'shell'`, mirroring terminal-ui.js's reasoning: in a plain shell it is the user's own job-control tool, everywhere else it silently suspends an unattended agent loop. Shift/Ctrl+Enter POSTs to `/api/sessions/:id/send-key` (`{key:'S-Enter'|'C-Enter'}`, tmux `send-keys -H` for a real 0x0a, or the CLI's declared `capabilities.newline` chord) targeting THIS pane's own `sessionId` rather than the primary pane's `activeSessionId` — without it xterm's plain `\r` would submit an incomplete prompt instead of adding a line to it. Smart-copy Ctrl+C/Ctrl+Shift+C is re-implemented against `this.terminal` (Pane B's own) rather than reusing `app.copyTerminalSelection()`, which reads Pane A's terminal and would copy the wrong pane's selection; Ctrl+Shift+C never falls through even with nothing to copy, mirroring terminal-ui.js's own `ev.shiftKey` branch. ⚠️ **This is a UX-parity fix, not an interrupt-safety one** — verified live in a real browser: xterm's `evaluateKeyboardEvent` routes a shifted ctrl-letter into a branch that assigns `c.key` only for two special cases (`_`→US, `@`→NUL), so it emits no data for Ctrl+Shift+C at all regardless of any application gate; a synthetic keydown with the gate removed produces zero WS frames, proving no accidental interrupt reaches the PTY either way. What gating the whole copy block on `hasSelection()` (an earlier draft) actually cost: with no selection, a selLine truncated
+**Split-pane sessions** (`showSplitButton`, header button, default OFF, per-device like `showFileViewerButton` — not in `SettingsUpdateSchema`, `displayKeys` in settings-ui.js): shows two live sessions side-by-side in one Codeman window. Pane A is the untouched, existing singleton terminal (`this.terminal`/`this._ws` in terminal-ui.js); Pane B is a new, independent `SplitTerminalPane` (terminal-split.js) with its own xterm instance and its own `/ws/sessions/:id/terminal` WebSocket. ⚠️ **Pane B is deliberately plainer than Pane A** — no local-echo overlay, no CJK IME, no touch/mobile handlers, no keyboard accessory bar — since this is a desktop-only feature (a split view needs a wide viewport) and those features exist for mobile/touch input; `.btn-split` is hard-hidden below 1180px regardless of the setting by the `@media (max-width: 1179px)` rule in styles.css (mobile.css only carries a comment pointing at it: that file loads up to 1023px, so it cannot cover the 1024-1179px tablet range the feature also needs to stay off), and the per-device setting means turning it on at a desk can never sync it onto a phone in the first place. No persistence: closing the browser tab or reloading always returns to the normal single-pane view; there is no localStorage key for split state. ⚠️ Splitting a session against itself is disallowed (the picker excludes the active session), **as is splitting against a popped-out (detached) session** — `buildSplitPickerSessions()` excludes `detachedSessions` because a detached session's own window is already claiming its PTY size, and `MAX_WS_PER_SESSION` needs no change since Pane A/B are always two different sessions. ⚠️ Either pane's session ending (deleted locally or from another client) auto-collapses the split — Pane A's session ending promotes Pane B to the new single pane via `selectSession(id, { auto: true })` (an app-driven selection, so it must not spend the session's idle alert — see the Approvals Inbox note above), never by trying to hot-swap the lightweight `SplitTerminalPane` object into the primary singleton state. ⚠️ Pane B refits on every window/sidebar/tab-rail resize via the SAME trailing-edge `ResizeObserver` callback that resizes Pane A (`throttledResize` in terminal-ui.js) — it only ever measured Pane A's own container, so without an explicit `this._splitPane?.fit()` call there Pane B silently kept its stale PTY size through every resize that did not happen to be a divider drag. ⚠️ A dropped WebSocket leaves Pane B visibly dead (a message written into its own xterm buffer) rather than silently swallowing keystrokes with nothing on screen to explain why — there is no reconnect logic for v1, matching the "deliberately plainer than Pane A" design. Related but distinct: `detachSession()` already opens one session in a separate OS-level browser window (`isSoloWindow`) — that is prior art for "two sessions visible at once" but not for one window with a draggable in-page divider, which is what this feature adds. ⚠️ Pane B installs its own `attachCustomKeyEventHandler` gating the same app-level chords Pane A's own handler gates (command palette, Alt+1-9/[/] tab nav, Alt+B sidebar toggle, Ctrl+Z suspend, Shift/Ctrl+Enter newline, smart-copy Ctrl+C) — without it the document capture-phase handler's `preventDefault()` (which never stops xterm) let each chord ALSO write its raw byte/escape sequence into Pane B's live PTY on top of whatever the app action did to Pane A (COD-153). Ctrl+Z is swallowed unless Pane B's own session is `mode === 'shell'`, mirroring terminal-ui.js's reasoning: in a plain shell it is the user's own job-control tool, everywhere else it silently suspends an unattended agent loop. Shift/Ctrl+Enter POSTs to `/api/sessions/:id/send-key` (`{key:'S-Enter'|'C-Enter'}`, tmux `send-keys -H` for a real 0x0a, or the CLI's declared `capabilities.newline` chord) targeting THIS pane's own `sessionId` rather than the primary pane's `activeSessionId` — without it xterm's plain `\r` would submit an incomplete prompt instead of adding a line to it. Smart-copy Ctrl+C/Ctrl+Shift+C is re-implemented against `this.terminal` (Pane B's own) rather than reusing `app.copyTerminalSelection()`, which reads Pane A's terminal and would copy the wrong pane's selection; Ctrl+Shift+C never falls through even with nothing to copy, mirroring terminal-ui.js's own `ev.shiftKey` branch. ⚠️ **This is a UX-parity fix, not an interrupt-safety one** — verified live in a real browser: xterm's `evaluateKeyboardEvent` routes a shifted ctrl-letter into a branch that assigns `c.key` only for two special cases (`_`→US, `@`→NUL), so it emits no data for Ctrl+Shift+C at all regardless of any application gate; a synthetic keydown with the gate removed produces zero WS frames, proving no accidental interrupt reaches the PTY either way. What gating the whole copy block on `hasSelection()` (an earlier draft) actually cost: with no selection, a selLine truncated
 
 ### Gesture control: the setting