diff --git a/docs/architecture-invariants.md b/docs/architecture-invariants.md
index 2e4e7039..a4b2e076 100644
--- a/docs/architecture-invariants.md
+++ b/docs/architecture-invariants.md
@@ -404,7 +404,7 @@ Anatomy: `.set-shell` → `.set-shell-head` (title + `.set-head-actions`) + `.se
### 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 on phones in mobile.css regardless of the setting, 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. Design: `docs/split-pane-sessions-plan.md`.
+**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 on phones in mobile.css regardless of the setting, 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 three app-level chords Pane A's own handler gates (command palette, Alt+1-9/[/] tab nav, Alt+B sidebar toggle) — 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+V stays on xterm's own default paste, since Pane B has no image-paste trap to route it to. Pane B's input frames deliberately carry no `cid`/`seq` (`ws-routes.ts` supports that), matching the no-overlay/no-IME "deliberately plainer" list above, since it has no exactly-once delivery layer to key them against. Design: `docs/split-pane-sessions-plan.md`.
### Gesture control: the setting
diff --git a/src/web/public/i18n.js b/src/web/public/i18n.js
index af1554ff..9a496a92 100644
--- a/src/web/public/i18n.js
+++ b/src/web/public/i18n.js
@@ -73,6 +73,10 @@
'File Viewer': '文件查看器',
'Open file viewer': '打开文件查看器',
'Open Codeman across all displays': '在所有显示器上打开 {name}',
+ 'Split: open a second session beside this one': '分屏:在旁边打开第二个会话',
+ 'Split: close the second session': '分屏:关闭第二个会话',
+ 'Close split': '关闭分屏',
+ 'No other sessions to split with': '没有其他可用于分屏的会话',
'Ultracode / Workflow agents': 'Ultracode / Workflow 智能体',
'Open ultracode workflow agents': '打开 Ultracode 工作流智能体',
Notifications: '通知',
diff --git a/src/web/public/index.html b/src/web/public/index.html
index 4c970475..dd972edc 100644
--- a/src/web/public/index.html
+++ b/src/web/public/index.html
@@ -1925,7 +1925,7 @@
-
+
diff --git a/src/web/public/styles.css b/src/web/public/styles.css
index 0e554240..74f223fc 100644
--- a/src/web/public/styles.css
+++ b/src/web/public/styles.css
@@ -675,6 +675,12 @@ body.tab-rail-resizing * {
user-select: none !important;
}
+body.split-pane-resizing,
+body.split-pane-resizing * {
+ cursor: col-resize !important;
+ user-select: none !important;
+}
+
@media (prefers-reduced-motion: reduce) {
.tab-rail,
.tab-rail-resize-handle {
@@ -18662,6 +18668,12 @@ html[data-session-list="sidebar"][data-sidebar="collapsed"] .btn-sidebar-toggle
}
.terminal-pane-b-close {
+ /* A native