Merge pull request #453 from timkjr/feat/split-pane-sessions

feat: split-pane sessions — view two live terminals side by side
2026-10-04 14:39:42 +02:00 · 2026-09-21 04:23:14 +02:00
parent ecb95b5d67 46d8b92049
commit 9a2e14a93a
23 changed files with 2446 additions and 6 deletions
@@ -1516,8 +1516,57 @@ function terminalLogicalLine(buffer, row, cols, maxRows) {
  return { startRow, endRow, text, offsetToCell, cellToOffset };
 }

+// ═══════════════════════════════════════════════════════════════
+// Split-Pane Sessions — pure helpers (divider math, picker list)
+// ═══════════════════════════════════════════════════════════════
+
+// Desktop-only, same reasoning and same threshold as HOME_SESSIONS_MIN_WIDTH
+// (home-sessions.js): two 240px min-width panes plus the divider need ~486px,
+// which a phone or narrow tablet cannot give them, and the divider has no
+// touch handlers. A dedicated constant rather than reusing
+// HOME_SESSIONS_MIN_WIDTH directly — that name lives in home-sessions.js,
+// which loads AFTER this file (load order 12.56 vs 7.5), so referencing it
+// from module-evaluation-time code here would be a ReferenceError.
+const SPLIT_PANE_MIN_WIDTH = 1180;
+
+function clampDividerPercent(rawPercent, min = 20, max = 80) {
+  if (rawPercent < min) return min;
+  if (rawPercent > max) return max;
+  return rawPercent;
+}
+
+function buildSplitPickerSessions(sessions, sessionOrder, excludeId, detachedIds) {
+  const result = [];
+  for (const id of sessionOrder) {
+    if (id === excludeId) continue;
+    // A detached (popped-out) session's own window already yields its PTY
+    // size (see sendResize's detachedElsewhere guard in terminal-ui.js) —
+    // Pane B's SplitTerminalPane._sendResize() has no such check, so letting
+    // one into the picker put its detached window and Pane B in a fight over
+    // the same PTY's dimensions.
+    if (detachedIds?.has?.(id)) continue;
+    const session = sessions.get(id);
+    if (!session) continue;
+    // A session with no PTY attached (exited CLI, a crash-looped session
+    // whose breaker tripped, a restore that failed to re-attach) has nothing
+    // reading its tmux pane. SplitTerminalPane never does selectSession()'s
+    // re-attach POST, so its socket would open onto a pane nothing feeds:
+    // no terminal events, and Session.write() silently drops every keystroke
+    // with no ack either way (Pane B sends no `seq`), so the loss is
+    // invisible — the healthy socket never trips the disconnect banner.
+    if (session.pid === null) continue;
+    result.push({ id, label: session.name || 'Session' });
+  }
+  return result;
+}
+
 if (typeof window !== 'undefined') {
  window.CodemanHistoryFormat = { formatHistoryBytes, computeHistoryTruncationNotice, computeRewriteScrollLine };
  window.CodemanFilePaths = { absoluteFilePathPattern, previewsInFileViewer, FILE_PREVIEW_EXTENSIONS };
  window.CodemanTerminalLines = { terminalLogicalLine };
+  window.CodemanSplitPane = {
+    clampDividerPercent,
+    buildSplitPickerSessions,
+    SPLIT_PANE_MIN_WIDTH,
+  };
 }
@@ -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',
@@ -191,6 +191,7 @@
        </button>
        <button class="btn-icon-header btn-file-viewer" onclick="app.toggleFileBrowserButton()" title="File Viewer" aria-label="Open file viewer" aria-expanded="false"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/></svg></button>
        <button class="btn-icon-header btn-multimonitor btn-multimonitor--hidden" onclick="app.launchMultiMonitor()" title="Open Codeman across all displays" aria-label="Open Codeman across all displays"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg></button>
+        <button class="btn-icon-header btn-split btn-split--hidden" onclick="app.openSplitPicker(event)" title="Split: open a second session beside this one" aria-label="Split: open a second session beside this one" aria-pressed="false"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="3" width="20" height="18" rx="2"/><line x1="12" y1="3" x2="12" y2="21"/></svg></button>
        <button class="btn-icon-header btn-ultracode-agents btn-ultracode-agents--hidden" onclick="app.toggleUltracodeAgentsPanel()" title="Ultracode / Workflow agents" aria-label="Open ultracode workflow agents"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="6" cy="6" r="2.5"/><circle cx="6" cy="18" r="2.5"/><circle cx="18" cy="12" r="2.5"/><path d="M8.2 7.2 15.6 11M8.2 16.8 15.6 13"/></svg></button>
        <div class="header-plan-usage header-plan-usage--hidden" id="planUsageChip" title="Claude and Codex plan usage limits">—</div>
        <button class="btn-icon-header btn-notifications" onclick="app.toggleNotifications()" title="Notifications" aria-label="Toggle notifications" style="display:none;">
@@ -1924,6 +1925,7 @@
                    <label class="set-chip" data-preview="header" data-preview-order="9"><input type="checkbox" id="appSettingsShowAttachmentsButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"/></svg><span>Attachments</span></label>
                    <label class="set-chip" data-preview="header" data-preview-order="10"><input type="checkbox" id="appSettingsShowFileViewerButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/></svg><span>File Viewer</span></label>
                    <label class="set-chip" data-preview="header" data-preview-order="11"><input type="checkbox" id="appSettingsShowMultiMonitorButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg><span>Multi-monitor</span></label>
+                    <label class="set-chip" data-preview="header" data-preview-order="11.5"><input type="checkbox" id="appSettingsShowSplitButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="3" width="20" height="18" rx="2"/><line x1="12" y1="3" x2="12" y2="21"/></svg><span>Split</span></label>
                    <label class="set-chip" data-preview="header" data-preview-order="13" data-preview-text="42%"><input type="checkbox" id="appSettingsShowPlanUsageLimits"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M4 18a8 8 0 1 1 16 0"/><path d="M12 18l4.5-5"/></svg><span>Plan Usage</span></label>
                    <label class="set-chip" data-preview="header" data-preview-order="14"><input type="checkbox" id="appSettingsShowLifecycleLog"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><line x1="16" y1="13" x2="8" y2="13"/><line x1="16" y1="17" x2="8" y2="17"/></svg><span>Lifecycle Log</span></label>
                  </div>
@@ -3719,6 +3721,7 @@
  <script defer src="app.js"></script>
  <script defer src="tab-rail-resize.js"></script>
  <script defer src="terminal-ui.js"></script>
+  <script defer src="terminal-split.js"></script>
  <script defer src="respawn-ui.js"></script>
  <script defer src="ralph-panel.js"></script>
  <script defer src="orchestrator-panel.js"></script>
@@ -529,6 +529,11 @@ html.mobile-init .file-browser-panel {
  .btn-icon-header.btn-approvals {
    display: none !important;
  }
+  /* Split-pane is hidden here too, but the AUTHORITATIVE hard gate is the
+     wider `@media (max-width: 1179px)` rule in styles.css — this file only
+     loads up to 1023px, which does not cover the 1024-1179px tablet range
+     the split feature also needs to stay off (two 240px min-width panes plus
+     the divider need ~486px; the divider also has no touch handlers). */

  /* Read My Mind 🧠 header button: never in the phone header; the phone
     surface is the keyboard-accessory 🧠 key (same `readMyMindEnabled` gate,
@@ -407,6 +407,7 @@ Object.assign(CodemanApp.prototype, {
    document.getElementById('appSettingsUltracodeFloatingWindows').checked =
      settings.ultracodeFloatingWindows ?? defaults.ultracodeFloatingWindows ?? false;
    document.getElementById('appSettingsShowMultiMonitorButton').checked = settings.showMultiMonitorButton ?? defaults.showMultiMonitorButton ?? false;
+    document.getElementById('appSettingsShowSplitButton').checked = settings.showSplitButton ?? defaults.showSplitButton ?? false;
    document.getElementById('appSettingsShowPlanUsageLimits').checked = this.planUsageChipEnabled(settings);
    document.getElementById('appSettingsShowRedrawButton').checked = settings.showRedrawButton ?? defaults.showRedrawButton ?? false;
    // Phone overview home screen: only meaningful under 600px, so the row is
@@ -2120,6 +2121,7 @@ Object.assign(CodemanApp.prototype, {
      readMyMindEnabled: document.getElementById('appSettingsReadMyMind').checked,
      ultracodeFloatingWindows: document.getElementById('appSettingsUltracodeFloatingWindows').checked,
      showMultiMonitorButton: document.getElementById('appSettingsShowMultiMonitorButton').checked,
+      showSplitButton: document.getElementById('appSettingsShowSplitButton').checked,
      showPlanUsageLimits: document.getElementById('appSettingsShowPlanUsageLimits').checked,
      showRedrawButton: document.getElementById('appSettingsShowRedrawButton').checked,
      mobileOverviewEnabled: document.getElementById('appSettingsMobileOverview').checked,
@@ -2350,6 +2352,10 @@ Object.assign(CodemanApp.prototype, {
      showPlanUsageLimits: _pul,
      showAttachmentsButton: _ahb,
      showFileViewerButton: _fvb,
+      // Desktop-only header button, per-device, and absent from
+      // SettingsUpdateSchema (.strict()) — sending it 400s the whole PUT
+      // (moving it into displayKeys alone is not the strip; this is).
+      showSplitButton: _ssp,
      webglRendererEnabled: _wgl,
      terminalWheelLocalScrollback: _twls,
      // Copy-on-select. Per-device (clipboard access differs by device and by
@@ -2742,6 +2748,7 @@ Object.assign(CodemanApp.prototype, {
        showUltracodeAgents: false,
        ultracodeFloatingWindows: false,
        showMultiMonitorButton: false,
+        showSplitButton: false,
        // Desktop defaults this ON (see planUsageChipEnabled); handhelds keep it
        // OFF so the phone header stays minimal and the mobile-header-buttons
        // policy guard keeps passing.
@@ -2947,6 +2954,13 @@ Object.assign(CodemanApp.prototype, {
      multiMonitorBtn.classList.toggle('btn-multimonitor--hidden', !showMultiMonitorButton);
    }

+    // Split button — hidden by default, and hard-gated to desktop widths
+    // regardless of the setting (window.CodemanSplitPane.SPLIT_PANE_MIN_WIDTH,
+    // matching HOME_SESSIONS_MIN_WIDTH's JS-check + media-query-backstop
+    // pattern — the CSS in styles.css is the backstop, this is the check).
+    const showSplitButton = settings.showSplitButton ?? defaults.showSplitButton ?? false;
+    this._applySplitButtonVisibility?.(showSplitButton);
+
    // Ultracode/Workflow agents launcher — hidden by default; reveal when enabled.
    // Marker class only (base is display:inline-flex !important) so it's auto-excluded
    // from the mobile-header-buttons-policy guard.
@@ -3363,6 +3377,7 @@ Object.assign(CodemanApp.prototype, {
          'showTabDetachButton',
          'mobileOverviewEnabled',
          'sessionLineageLines',
+          'showSplitButton',
        ]);
        // The plan-usage chip is a PER-DEVICE display setting (desktop default ON,
        // handheld default OFF): desktop can show it while mobile stays hidden. Drop
@@ -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 {
@@ -2282,6 +2288,39 @@ html[data-tab-orientation='vertical'] .tab-rail .session-tab .tab-name-prefix {
  display: none !important;
 }

+/* Split-pane header button: hidden by default (opt-in via App Settings →
+   Header & Panels → Header buttons → Split, `showSplitButton`). Pure
+   client-side toggle applied by applyHeaderVisibilitySettings(). */
+.btn-split--hidden {
+  display: none !important;
+}
+
+/* While a split is open, the button's own click closes it instead of opening
+   the picker — the accent fill is what tells the user which of its two
+   behaviours the next click will get (aria-pressed carries the same state
+   for assistive tech). */
+.btn-icon-header.btn-split.split-open {
+  background: var(--accent);
+  color: var(--accent-ink);
+}
+
+.btn-icon-header.btn-split.split-open:hover {
+  background: var(--accent);
+  color: var(--accent-ink);
+}
+
+/* Split-pane hard desktop gate, independent of the showSplitButton setting:
+   two 240px min-width panes plus the divider need ~486px, the divider has no
+   touch handlers, and mobile.css only loads up to 1023px so it cannot cover
+   the 1024-1179px tablet range on its own. Same threshold and reasoning as
+   HOME_SESSIONS_MIN_WIDTH (home-sessions.js) / SPLIT_PANE_MIN_WIDTH
+   (constants.js) — keep the three in sync. */
+@media (max-width: 1179px) {
+  .btn-icon-header.btn-split {
+    display: none !important;
+  }
+}
+
 .btn-icon-header.btn-settings {
  width: 30px;
  height: 30px;
@@ -2547,6 +2586,7 @@ body.solo-mode .header-system-stats,
 body.solo-mode .header-tokens,
 body.solo-mode .btn-notifications,
 body.solo-mode .btn-multimonitor,
+body.solo-mode .btn-split,
 body.solo-mode .header-plan-usage,
 /* A solo window shows ONE session and has no tab strip to put restored ones in,
   so offering to rebuild a list of them there is an offer it cannot show the
@@ -15299,6 +15339,13 @@ html[data-skin="daylight-blue"] .welcome-btn-tunnel.active:hover {
 }
 .main.webview-active .webview-layer { display: flex; }
 .main.webview-active .terminal-wrap { display: none; }
+/* A split (Pane A + divider + Pane B) hides as one unit when a web tab is
+   active, mirroring the .terminal-wrap rule above — .terminal-wrap is
+   reparented INSIDE .terminal-split-container while a split is open, so
+   hiding only .terminal-wrap would leave Pane B and the divider stranded on
+   screen over the dashboard iframe. No state is destroyed, so returning to
+   the session tab shows the split intact. */
+.main.webview-active .terminal-split-container { display: none; }

 .webview-frame {
  display: none;
@@ -18565,3 +18612,128 @@ html[data-session-list="sidebar"][data-sidebar="collapsed"] .btn-sidebar-toggle
    max-height: min(88vh, env(viewport-segment-height 0 1, 88vh));
  }
 }
+
+/* Split-Pane Sessions: container inserted only while a split is active.
+   .terminal-wrap (Pane A) is reparented into this as the first child; it
+   keeps every existing rule unchanged since nothing here restyles it. */
+.terminal-split-container {
+  display: flex;
+  flex-direction: row;
+  width: 100%;
+  height: 100%;
+  min-height: 0;
+}
+
+.terminal-split-container > .terminal-wrap {
+  /* flex-shrink 1 (not 0): the divider's own 6px is fixed-width, and Pane A
+     +Pane B's inline flex-basis (openSplitPane/onMove) always sums to 100%,
+     so with flex-shrink 0 on both panes the row is 100% + 6px wide and
+     .main's overflow clips Pane B's right edge by exactly the divider's
+     width. Shrinking lets the two panes give up that 6px between them. */
+  flex: 0 1 auto;
+  min-width: 240px;
+  overflow: hidden;
+}
+
+.split-divider {
+  flex: 0 0 6px;
+  cursor: col-resize;
+  background: var(--border-color, #333);
+  position: relative;
+}
+
+.split-divider:hover,
+.split-divider.dragging {
+  background: var(--accent-color, #4a9eff);
+}
+
+.terminal-pane-b {
+  /* flex-shrink 1, matching .terminal-wrap above — see its comment. */
+  flex: 0 1 auto;
+  min-width: 240px;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+}
+
+.terminal-pane-b-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 4px 8px;
+  font-size: 12px;
+  background: var(--bg-secondary, #1a1a1a);
+  border-bottom: 1px solid var(--border-color, #333);
+  flex: 0 0 auto;
+}
+
+.terminal-pane-b-close {
+  /* A native <button> now backs this (keyboard-reachable close), so reset its
+     default chrome back to the plain glyph this rule always drew. */
+  border: none;
+  background: none;
+  font: inherit;
+  color: inherit;
+  cursor: pointer;
+  padding: 0 6px;
+  opacity: 0.7;
+}
+
+.terminal-pane-b-close:hover {
+  opacity: 1;
+}
+
+.terminal-pane-b-container {
+  flex: 1 1 auto;
+  min-height: 0;
+}
+
+/* Split-picker menu: a small popover listing sessions to split with, appended
+   to document.body and positioned `fixed` by openSplitPicker() (JS sets
+   top/right against the Split header button's own rect). z-index above the
+   header (100) with headroom to spare, matching the sibling .run-mode-menu
+   dropdown's 1000. Dismissed by openSplitPane()'s own inline onclick, or by
+   the click-outside/Escape listeners installed alongside it. */
+.split-picker-menu {
+  position: fixed;
+  z-index: 1000;
+  min-width: 200px;
+  max-width: 320px;
+  padding: 4px;
+  background: var(--floating-bg);
+  backdrop-filter: blur(20px);
+  -webkit-backdrop-filter: blur(20px);
+  border: 1px solid var(--control-border);
+  border-radius: 10px;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.5), 0 2px 8px rgba(0, 0, 0, 0.3);
+}
+
+.split-picker-item {
+  /* A native <button> now backs each row (keyboard-reachable picker), so
+     reset its default chrome back to the plain list-row look this always
+     drew — full-width, left-aligned, no border/background of its own. */
+  display: block;
+  width: 100%;
+  text-align: left;
+  border: none;
+  background: none;
+  font: inherit;
+  color: inherit;
+  padding: 8px 10px;
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: 13px;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.split-picker-item:hover {
+  background: var(--control-bg-hover, rgba(255, 255, 255, 0.08));
+}
+
+.split-picker-empty {
+  padding: 8px 10px;
+  font-size: 13px;
+  color: var(--text-muted);
+}
@@ -0,0 +1,706 @@
+// src/web/public/terminal-split.js
+
+/**
+ * @fileoverview SplitTerminalPane — a second, independent live terminal pane
+ * ("Pane B") for split-view sessions. Deliberately plainer than the primary
+ * pane (this.terminal/this._ws in terminal-ui.js): no local-echo overlay, no
+ * CJK IME, no touch/mobile handlers, no keyboard accessory bar. Desktop-only
+ * feature by nature — see docs/split-pane-sessions-plan.md.
+ *
+ * @dependency vendor/xterm.js, vendor/xterm-addon-fit.js
+ * @dependency constants.js (window.CodemanTerminalFont, DEFAULT_SCROLLBACK, TERMINAL_TAIL_SIZE, TERMINAL_CHUNK_SIZE)
+ * @dependency terminal-ui.js (codemanCurrentXtermTheme, codemanCurrentSkinIsLight)
+ * @loadorder 7.5 of 16 — loaded after terminal-ui.js, before respawn-ui.js
+ */
+
+(function (global) {
+  /**
+   * Minimal chunked write for Pane B's own xterm instance — write() in
+   * TERMINAL_CHUNK_SIZE slices, yielding a frame between each, instead of one
+   * giant synchronous write that blocks the main thread while parsing a long
+   * scrollback. Deliberately NOT the primary pane's chunkedTerminalWrite
+   * (terminal-ui.js): that one is wired into session-switch generation
+   * counters and the live-output gate this simpler, independently
+   * created/destroyed pane has no equivalent of.
+   */
+  function writeChunked(terminal, buffer, isDestroyed) {
+    if (!buffer) return;
+    if (buffer.length <= TERMINAL_CHUNK_SIZE) {
+      terminal.write(buffer);
+      return;
+    }
+    let offset = 0;
+    const writeNext = () => {
+      if (isDestroyed() || !terminal) return;
+      const chunk = buffer.slice(offset, offset + TERMINAL_CHUNK_SIZE);
+      offset += chunk.length;
+      terminal.write(chunk);
+      if (offset < buffer.length) {
+        if (typeof requestAnimationFrame === 'function') requestAnimationFrame(writeNext);
+        else setTimeout(writeNext, 16);
+      }
+    };
+    writeNext();
+  }
+
+  class SplitTerminalPane {
+    constructor(sessionId, mountEl, opts = {}) {
+      this.sessionId = sessionId;
+      this.mountEl = mountEl;
+      this.sessionMode = opts.mode;
+      this.fontSettings = opts.fontSettings || {};
+      // Live reference (not a snapshot) to the app's detachedSessions Set —
+      // detaching this session AFTER the split is already open must still be
+      // seen by _sendResize() below, or it re-creates the exact PTY-size
+      // fight the split picker already refuses to open at pick time.
+      this.detachedSessions = opts.detachedSessions;
+      this.terminal = null;
+      this.fitAddon = null;
+      this.ws = null;
+      this._wsReady = false;
+      this._destroyed = false;
+    }
+
+    async connect() {
+      const savedFontSize = parseInt(localStorage.getItem('codeman-font-size'), 10);
+      this.terminal = new Terminal({
+        theme: { ...global.codemanCurrentXtermTheme() },
+        fontFamily: global.CodemanTerminalFont.resolve(this.fontSettings.terminalFontFamily),
+        ...global.CodemanTerminalFont.resolveWeights(this.fontSettings),
+        fontSize: Number.isFinite(savedFontSize) ? savedFontSize : 14,
+        lineHeight: 1.2,
+        cursorBlink: false,
+        cursorStyle: 'block',
+        minimumContrastRatio: global.codemanCurrentSkinIsLight() ? 4.5 : 1,
+        scrollback: DEFAULT_SCROLLBACK,
+        allowTransparency: true,
+        allowProposedApi: true,
+      });
+
+      this.fitAddon = new FitAddon.FitAddon();
+      this.terminal.loadAddon(this.fitAddon);
+      this.terminal.open(this.mountEl);
+      this.fitAddon.fit();
+
+      this.terminal.onData((data) => {
+        if (this.ws && this.ws.readyState === WebSocket.OPEN) {
+          this.ws.send(JSON.stringify({ t: 'i', d: data }));
+        }
+      });
+
+      // Pane B has no gates of its own by default, so every app-level chord
+      // that the document capture-phase handler (app.js) only preventDefault()s
+      // — never stopPropagation()s — reaches xterm here too and writes its raw
+      // byte/escape sequence into THIS session's PTY on top of whatever the app
+      // action already did to Pane A (COD-153; mirrors the primary pane's own
+      // gates at terminal-ui.js's attachCustomKeyEventHandler: command palette,
+      // Alt+1-9/[/] tab nav, Alt+B sidebar toggle, Ctrl+Z suspend, Shift/Ctrl+Enter
+      // newline, and smart-copy Ctrl+C/Ctrl+Shift+C). Routed through the same
+      // registry-aware predicates so a rebind or a disable restores plain
+      // terminal behavior here too. Ctrl+V is deliberately left on xterm's own
+      // default (plain-text paste): Pane B has no image-paste trap to route it
+      // to, so intercepting it here would only break paste.
+      this.terminal.attachCustomKeyEventHandler((ev) => {
+        if (ev.isComposing || ev.key === 'Process' || ev.keyCode === 229) return true;
+        if (
+          ev.altKey &&
+          !ev.ctrlKey &&
+          !ev.shiftKey &&
+          /^(Digit[1-9]|BracketLeft|BracketRight|KeyK)$/.test(ev.code || '')
+        ) {
+          return false;
+        }
+        if (ev.type === 'keydown' && global.app?.shouldOpenCommandPaletteFromShortcut?.(ev)) {
+          return false;
+        }
+        if (ev.type === 'keydown' && global.app?.shouldToggleSessionSidebarFromShortcut?.(ev)) {
+          return false;
+        }
+        // Ctrl+Z (SIGTSTP/job-control suspend): mirrors terminal-ui.js's own
+        // swallow — in a plain shell session this is the user's own
+        // job-control tool and must reach the PTY, but in every other mode
+        // (claude/omp/pi/codex/...) it silently stops an unattended agent
+        // loop dead. Pane B has its own PTY/session and must not send a
+        // suspend into a non-shell one just because the primary pane's own
+        // gate lives elsewhere.
+        if (
+          ev.type === 'keydown' &&
+          ev.key.toLowerCase() === 'z' &&
+          ev.ctrlKey &&
+          !ev.altKey &&
+          !ev.metaKey &&
+          !ev.shiftKey &&
+          this.sessionMode !== 'shell'
+        ) {
+          return false;
+        }
+        // Shift+Enter / Ctrl+Enter: insert a newline instead of submitting.
+        // Mirrors terminal-ui.js's own handling — xterm sends plain \r for
+        // every Enter variant, so an Ink app (Claude Code) can't tell a
+        // newline from a submit. Without this gate, Pane B's onData would
+        // send that bare \r straight over the WS and submit an incomplete
+        // prompt instead of adding a line to it. Targets THIS pane's own
+        // session (this.sessionId), never the primary pane's
+        // activeSessionId, and has no local-echo overlay of its own to flush
+        // first (Pane B is deliberately plainer — see the fileoverview).
+        if (ev.key === 'Enter' && (ev.shiftKey || ev.ctrlKey) && ev.type === 'keydown') {
+          fetch(`/api/sessions/${this.sessionId}/send-key`, {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify({ key: ev.ctrlKey ? 'C-Enter' : 'S-Enter' }),
+          }).catch(() => {
+            /* Best-effort, matching this pane's tolerance elsewhere. */
+          });
+          return false;
+        }
+        // Smart copy (mirrors terminal-ui.js's Ctrl+C gate, #211): with a
+        // selection, Ctrl+C copies THIS pane's own selection instead of
+        // sending ^C; with none, plain Ctrl+C must fall through unchanged or
+        // the interrupt key is lost. Ctrl+Shift+C is different: it is the
+        // explicit, never-falls-through copy chord, and the predicate above
+        // does not distinguish it from plain Ctrl+C — ev.shiftKey does, below.
+        // xterm's own evaluateKeyboardEvent routes a shifted ctrl-letter into
+        // a branch that assigns c.key only for a couple of special cases
+        // ("_"->US, "@"->NUL), neither of which is "c", so it emits NOTHING
+        // for Ctrl+Shift+C either way — this is not about an accidental
+        // interrupt byte reaching the PTY (verified live: it does not).
+        // Gating this whole block on hasSelection() (an earlier draft) meant
+        // that with no selection Ctrl+Shift+C skipped straight to `return
+        // true`, silently ceding the keystroke to the BROWSER's own handling
+        // (e.g. Chrome's Inspect-Element binding) with no feedback and no
+        // attempt to copy, unlike Pane A, which always intercepts it.
+        // Re-implemented against this.terminal rather than reusing
+        // app.copyTerminalSelection(), which reads app.terminal — Pane A's —
+        // and would copy the wrong pane's selection.
+        if (ev.type === 'keydown' && global.app?.shouldCopyTerminalSelectionFromShortcut?.(ev)) {
+          const raw = this.terminal?.getSelection?.() || '';
+          const isColumnSelection = this.terminal?._core?._selectionService?._activeSelectionMode === 3;
+          const selection = isColumnSelection ? raw : (global.CodemanCopySelection?.clean?.(raw) ?? raw);
+          if (selection.trim()) {
+            ev.preventDefault();
+            void global.app._copyText?.(selection).then((ok) => {
+              this.terminal?.clearSelection?.();
+              global.app.showToast?.(ok ? 'Copied to clipboard' : 'Failed to copy', ok ? 'success' : 'error');
+            });
+            return false;
+          }
+          // Nothing worth copying — clear for feedback (a padding-only
+          // selection cleans to '' and this press still falls through to the
+          // PTY as 0x03, matching the primary pane's own rule).
+          if (this.terminal?.hasSelection?.()) {
+            this.terminal.clearSelection?.();
+            global.app.showToast?.('Nothing to copy', 'warning');
+          }
+          // Ctrl+Shift+C never falls through, even with nothing to copy —
+          // matches terminal-ui.js's own ev.shiftKey branch.
+          if (ev.shiftKey) {
+            ev.preventDefault();
+            return false;
+          }
+        }
+        return true;
+      });
+
+      // Load existing scrollback before going live. The WS below is
+      // subscribe-only (ws-routes.ts sends nothing on connect, only future
+      // 'terminal' events), so without this Pane B stays blank until the
+      // target session happens to produce new output. It LOOKED
+      // intermittent rather than always-broken because _sendResize() below
+      // often nudges the shared session's real tmux window to a new size,
+      // and tmux repaints its current screen on resize — that repaint was
+      // getting captured and streamed here, incidentally populating the
+      // pane. When Pane B's computed dimensions happened to already match
+      // the session's last-known size, Session.resize() (session.ts) skips
+      // the resize as a no-op, no repaint fires, and the pane stayed blank.
+      await this._loadBuffer();
+      if (this._destroyed) return;
+
+      const proto = location.protocol === 'https:' ? 'wss:' : 'ws:';
+      const url = `${proto}//${location.host}${window.CodemanBase.base}/ws/sessions/${this.sessionId}/terminal`;
+      this.ws = new WebSocket(url);
+
+      this.ws.onopen = () => {
+        this._wsReady = true;
+        this._sendResize();
+      };
+
+      this.ws.onmessage = (event) => {
+        try {
+          const msg = JSON.parse(event.data);
+          if (msg.t === 'o') {
+            this.terminal.write(msg.d);
+          } else if (msg.t === 'c') {
+            this.terminal.clear();
+          } else if (msg.t === 'r') {
+            // Server-triggered refresh (SSE backpressure cleared, terminal
+            // data was dropped). The primary pane routes this to
+            // _onSessionNeedsRefresh (app.js:2990) — Pane B has its own
+            // buffer loader for the same reason connect() does.
+            this.terminal.clear();
+            void this._loadBuffer();
+          }
+        } catch {
+          /* Malformed frame — ignore, matches primary pane's tolerance. */
+        }
+      };
+
+      // Mirror app.js's onclose/onerror pattern (app.js:2905-2964): _wsReady
+      // must go false on a drop or fit()/_sendResize() silently no-ops on a
+      // closed socket per the WebSocket spec (no exception, no log). No
+      // reconnect logic here — Pane B is deliberately plainer than the
+      // primary pane (see the fileoverview above); a drop just stops
+      // resizing until the parent recreates the pane. But onData already
+      // silently drops keystrokes while _wsReady is false (below), so
+      // without a visible marker a dropped socket left Pane B looking
+      // normal while it quietly ate everything typed into it. v1 scope is
+      // "say so", not reconnect — collapsing the split would lose the
+      // user's place in Pane B's scrollback for a transient blip.
+      this.ws.onclose = () => {
+        this._wsReady = false;
+        this.terminal?.write('\r\n\x1b[2m[Pane B disconnected — close and reopen the split to reconnect]\x1b[0m\r\n');
+      };
+
+      this.ws.onerror = () => {
+        // onclose fires after onerror — cleanup happens there.
+      };
+    }
+
+    // Fetches and writes the session's current scrollback. Used both by
+    // connect() (initial load) and by the `{t:'r'}` server-refresh frame
+    // (below) — the primary pane's own _onSessionNeedsRefresh (app.js) is
+    // scoped to `this.activeSessionId` and clears/rewrites the primary
+    // terminal, neither of which applies to this independent pane, so this is
+    // a standalone equivalent rather than a call into it.
+    //
+    // Mirrors the primary pane's own mode check (app.js's selectSession /
+    // _onSessionNeedsRefresh): a shell session can retain hundreds of
+    // thousands of plain scrollback lines, so pulling `?full=1` there parses
+    // an unbounded, server-capped (up to terminalBufferMaxBytes, 32MB) body
+    // into a 50000-line xterm on every load. Non-shell (TUI) sessions still
+    // get one full replay. `fetch` here goes through the global wrapper
+    // (constants.js), which already prefixes CodemanBase — unlike the raw
+    // WebSocket URL above, which does not.
+    async _loadBuffer() {
+      try {
+        const query = this.sessionMode === 'shell' ? `tail=${TERMINAL_TAIL_SIZE}` : 'full=1';
+        const res = await fetch(`/api/sessions/${this.sessionId}/terminal?${query}`);
+        const payload = (await res.json())?.data ?? {};
+        if (payload.terminalBuffer && this.terminal) {
+          writeChunked(this.terminal, payload.terminalBuffer, () => this._destroyed);
+        }
+      } catch {
+        /* Best-effort — live output still arrives once the socket connects. */
+      }
+    }
+
+    // Local reflow only — no PTY resize frame. Split out so a divider drag
+    // can reflow both panes at the browser's paint rate (rAF) while sending
+    // the actual `{t:'z'}` resize once, at drag end, matching the primary
+    // pane's own convention (throttledResize in terminal-ui.js).
+    localFit() {
+      if (!this.fitAddon) return;
+      this.fitAddon.fit();
+    }
+
+    fit() {
+      this.localFit();
+      this._sendResize();
+    }
+
+    _sendResize() {
+      if (!this._wsReady || !this.fitAddon) return;
+      // One PTY cannot hold two sizes (mirrors sendResize's own
+      // detachedElsewhere yield in terminal-ui.js): the session got detached
+      // to its own window AFTER this split was opened, so its own window now
+      // owns the PTY's size and Pane B must stand aside.
+      if (this.detachedSessions?.has(this.sessionId)) return;
+      const dims = this.fitAddon.proposeDimensions();
+      if (!dims) return;
+      // Send the real proposed dimensions unclamped, matching the primary
+      // pane's convention (terminal-ui.js's getTerminalDimensions()) — the
+      // server enforces its own valid range ([1,500]/[1,200] in ws-routes.ts).
+      // A 40/10 floor here misreported Pane B's real width to the PTY at the
+      // divider's own reachable 20% floor position, causing real
+      // output-wrapping bugs.
+      this.ws.send(JSON.stringify({ t: 'z', c: dims.cols, r: dims.rows, v: 'desktop' }));
+    }
+
+    destroy() {
+      this._destroyed = true;
+      if (this.ws) {
+        this.ws.onopen = null;
+        this.ws.onmessage = null;
+        this.ws.close();
+        this.ws = null;
+      }
+      if (this.terminal) {
+        this.terminal.dispose();
+        this.terminal = null;
+      }
+      this.fitAddon = null;
+    }
+  }
+
+  global.SplitTerminalPane = SplitTerminalPane;
+})(window);
+
+Object.assign(CodemanApp.prototype, {
+  /**
+   * Desktop-only gate, same shape as home-sessions.js's shouldShowHomeSessions
+   * + matchMedia backstop: a JS width check (so openSplitPane() below can
+   * refuse even if a click somehow reaches the button) plus a live listener,
+   * because a window narrowed WHILE the button is showing must hide it
+   * without waiting for a settings save or reload. The CSS `@media
+   * (max-width: 1179px)` rule in styles.css is the backstop for the reverse
+   * direction: it hides the button even if this JS never runs at all.
+   */
+  _applySplitButtonVisibility(enabled) {
+    this._splitButtonSettingEnabled = enabled;
+    const splitBtn = document.querySelector('.btn-split');
+    const wide = window.innerWidth >= SPLIT_PANE_MIN_WIDTH;
+    // Narrowing past the gate must not leave an open split on screen with no
+    // way to reach the button that would close it — the two 240px min-widths
+    // plus the divider overflow a narrow window and .main clips Pane B's edge.
+    if (!wide && this._splitPane) this.closeSplitPane();
+    if (!splitBtn) return;
+    splitBtn.classList.toggle('btn-split--hidden', !enabled || !wide);
+    if (!this._splitButtonWidthListenerInstalled && window.matchMedia) {
+      this._splitButtonWidthListenerInstalled = true;
+      const mq = window.matchMedia(`(min-width: ${SPLIT_PANE_MIN_WIDTH}px)`);
+      mq.addEventListener('change', () => this._applySplitButtonVisibility(this._splitButtonSettingEnabled));
+    }
+  },
+
+  openSplitPicker(event) {
+    // Mirrors toggleRunModeMenu (session-ui.js): stopPropagation on the
+    // OPENING click so it never reaches the outside-click listener this
+    // same call is about to register — without it, a click landing on the
+    // button's own inner <svg> (matched by neither `menu.contains()` nor
+    // the old exact-node check below) bubbled straight through to
+    // `document` and self-closed the menu it just opened.
+    event?.stopPropagation();
+    if (this._splitPane) {
+      this.closeSplitPane();
+      return;
+    }
+    const candidates = window.CodemanSplitPane.buildSplitPickerSessions(
+      this.sessions,
+      this.sessionOrder,
+      this.activeSessionId,
+      this.detachedSessions
+    );
+    // Route a pre-existing menu through the SAME dismiss path used
+    // everywhere else, instead of a raw `.remove()`: a genuinely still-open
+    // menu has live document listeners (see below), and a raw removal left
+    // them attached forever — only the single-slot field below got
+    // overwritten, so every prior pair but the last was orphaned on
+    // `document` with no way to ever find and remove it again.
+    this._dismissSplitPicker();
+
+    const menu = document.createElement('div');
+    menu.id = 'splitPickerMenu';
+    menu.className = 'split-picker-menu';
+    if (candidates.length === 0) {
+      menu.innerHTML = '<div class="split-picker-empty">No other sessions to split with</div>';
+    } else {
+      menu.innerHTML = candidates
+        .map(
+          (c) =>
+            // data-i18n-skip: the whole row's text IS a session name — i18n.js
+            // does exact-string lookup over text nodes, and a session
+            // literally named e.g. "Sessions" would otherwise get translated
+            // on zh-CN (see the .session-name skip on the pane header below).
+            `<button type="button" class="split-picker-item" data-i18n-skip data-session-id="${escapeHtml(c.id)}" onclick="app.openSplitPane(${escapeHtml(JSON.stringify(c.id))}); app._dismissSplitPicker();">${escapeHtml(c.label)}</button>`
+        )
+        .join('');
+    }
+    document.body.appendChild(menu);
+    const splitBtn = document.querySelector('.btn-split');
+    if (splitBtn) {
+      const rect = splitBtn.getBoundingClientRect();
+      menu.style.position = 'fixed';
+      menu.style.top = `${rect.bottom + 4}px`;
+      menu.style.right = `${window.innerWidth - rect.right}px`;
+    }
+
+    // Dismiss on outside click or Escape — same one-shot listener pattern as
+    // session-ui.js's other transient popovers (toggleCaseSettings(),
+    // toggleRunModeMenu()). Deferred by a tick so the click that OPENED the
+    // menu (still bubbling) doesn't immediately close it — reinforced by
+    // the button's own stopPropagation() above, which is what actually
+    // stops that same click reaching `document` at all. Picking an item
+    // (above) calls the SAME dismiss method, so these listeners never
+    // outlive the menu either way.
+    //
+    // Self-removing by identity: each handler removes ITSELF (and its
+    // sibling) the moment it fires, rather than leaning solely on the
+    // `this._splitPickerDismissHandlers` field. That field is still kept in
+    // sync (so `_dismissSplitPicker()` called from elsewhere — the picker
+    // item's onclick above, or a still-open menu at the top of this method
+    // — can find and remove the CURRENT pair), but no path here can ever
+    // again leave a pair attached to `document` with nothing referencing it.
+    const closeOnOutsideClick = (e) => {
+      if (menu.contains(e.target) || e.target.closest('.btn-split')) return;
+      document.removeEventListener('click', closeOnOutsideClick);
+      document.removeEventListener('keydown', closeOnEscape);
+      this._splitPickerDismissHandlers = null;
+      menu.remove();
+    };
+    const closeOnEscape = (e) => {
+      if (e.key !== 'Escape') return;
+      document.removeEventListener('click', closeOnOutsideClick);
+      document.removeEventListener('keydown', closeOnEscape);
+      this._splitPickerDismissHandlers = null;
+      menu.remove();
+    };
+    this._splitPickerDismissHandlers = { closeOnOutsideClick, closeOnEscape };
+    setTimeout(() => document.addEventListener('click', closeOnOutsideClick), 0);
+    document.addEventListener('keydown', closeOnEscape);
+  },
+
+  _dismissSplitPicker() {
+    document.getElementById('splitPickerMenu')?.remove();
+    if (this._splitPickerDismissHandlers) {
+      document.removeEventListener('click', this._splitPickerDismissHandlers.closeOnOutsideClick);
+      document.removeEventListener('keydown', this._splitPickerDismissHandlers.closeOnEscape);
+      this._splitPickerDismissHandlers = null;
+    }
+  },
+
+  openSplitPane(sessionId) {
+    // Desktop-only hard gate, independent of the button's own hidden state —
+    // see _applySplitButtonVisibility's comment for why both a JS check and
+    // a CSS backstop exist.
+    if (window.innerWidth < SPLIT_PANE_MIN_WIDTH) return;
+    // No active session means there is no `.terminal-wrap` to split against
+    // (the welcome overlay is showing) — without this, a split opened from
+    // the home screen still created the container and connected Pane B, just
+    // behind the opaque overlay with nothing visible to show for it.
+    if (!this.activeSessionId) return;
+    // A web tab hides `.terminal-wrap`'s container via CSS with nothing
+    // gating the button itself, and `activeSessionId` survives openWebview()
+    // — without this, picking a session opens Pane B's socket behind a
+    // hidden container with nothing on screen to show for it.
+    if (this.activeWebviewId) return;
+    // A stale picker click (opened before switching tabs) or clicking Pane
+    // B's own session tab while split can otherwise land here with
+    // sessionId === activeSessionId: two live WebSockets to the same
+    // session, each independently claiming PTY dimensions via its own `{t:'z',...}`
+    // resize frame. Refuse before creating any DOM or SplitTerminalPane.
+    if (sessionId === this.activeSessionId) return;
+    if (this._splitPane) this.closeSplitPane();
+
+    const wrap = document.querySelector('.terminal-wrap');
+    const parent = wrap.parentElement;
+
+    const container = document.createElement('div');
+    container.className = 'terminal-split-container';
+
+    const divider = document.createElement('div');
+    divider.className = 'split-divider';
+
+    const paneB = document.createElement('div');
+    paneB.className = 'terminal-pane-b';
+    const session = this.sessions.get(sessionId);
+    paneB.innerHTML = `
+      <div class="terminal-pane-b-header">
+        <span class="session-name">${escapeHtml(session?.name || 'Session')}</span>
+        <button type="button" class="terminal-pane-b-close" onclick="app.closeSplitPane()" aria-label="Close split">&times;</button>
+      </div>
+      <div class="terminal-pane-b-container"></div>
+    `;
+
+    parent.insertBefore(container, wrap);
+    container.appendChild(wrap);
+    wrap.style.flexBasis = '50%';
+    container.appendChild(divider);
+    container.appendChild(paneB);
+    paneB.style.flexBasis = '50%';
+
+    this._splitPane = new window.SplitTerminalPane(sessionId, paneB.querySelector('.terminal-pane-b-container'), {
+      mode: session?.mode,
+      fontSettings: this.loadAppSettingsFromStorage?.() || {},
+      detachedSessions: this.detachedSessions,
+    });
+    this._splitPane.connect().catch(() => {
+      /* Best-effort, matching the primary pane's own tolerance for a failed
+         initial load — live output still arrives once/if the socket connects. */
+    });
+    this._splitSessionId = sessionId;
+
+    // Pane A just went from full width to 50%, but nothing has told its
+    // session's PTY/tmux window about it yet — the passive ResizeObserver in
+    // terminal-ui.js debounces 300ms and would eventually catch up, but
+    // relying on that left the pane showing stale-width content (existing
+    // box-drawing lines, banners) until the user hit "Redraw Terminal".
+    // Force it immediately, mirroring closeSplitPane()'s symmetric call.
+    this.sendResize?.(this.activeSessionId, { force: true })?.catch?.(() => {});
+
+    this._installSplitDividerDrag(divider, wrap, paneB);
+    this._updateSplitButtonState(true);
+  },
+
+  closeSplitPane() {
+    if (!this._splitPane) return;
+    this._splitPane.destroy();
+    this._splitPane = null;
+    this._splitSessionId = null;
+    this._updateSplitButtonState(false);
+
+    const container = document.querySelector('.terminal-split-container');
+    if (!container) return;
+    const wrap = container.querySelector('.terminal-wrap');
+    const parent = container.parentElement;
+    wrap.style.flexBasis = '';
+    parent.insertBefore(wrap, container);
+    container.remove();
+
+    if (this.fitAddon) this.fitAddon.fit();
+    this.sendResize?.(this.activeSessionId, { force: true })?.catch?.(() => {});
+  },
+
+  // A click on .btn-split does one of two things — open the picker, or
+  // (openSplitPicker's own early return) close an already-open split — and
+  // nothing on the button said which. `.split-open` + aria-pressed give it
+  // the same active-state language as the codebase's other toggle buttons
+  // (keyboard-accessory's Ctrl key, the voice-input mic).
+  _updateSplitButtonState(open) {
+    const btn = document.querySelector('.btn-split');
+    if (!btn) return;
+    btn.classList.toggle('split-open', open);
+    btn.setAttribute('aria-pressed', open ? 'true' : 'false');
+    const title = open ? 'Split: close the second session' : 'Split: open a second session beside this one';
+    btn.title = title;
+    btn.setAttribute('aria-label', title);
+  },
+
+  _installSplitDividerDrag(divider, wrap, paneB) {
+    let dragging = false;
+    let dragRaf = null;
+    let pendingClientX = null;
+
+    // Local-only reflow (flexBasis + both panes' xterm fit, no PTY resize
+    // frame). Coalesced to one call per animation frame below — a raw
+    // mousemove stream fires far faster than the browser repaints, and
+    // without the rAF gate each event did a full xterm reflow on BOTH
+    // panes AND sent Pane B a `{t:'z'}` resize frame (SplitTerminalPane has
+    // no client-side "dims unchanged" skip), which fanned out into a
+    // `tmux resize-window` child plus a SIGWINCH per frame — roughly fifty
+    // of each dragging across half a wide viewport.
+    const applyDragPercent = (clientX) => {
+      const container = divider.parentElement;
+      // The split can auto-collapse mid-drag (the other pane's session
+      // ending, or the picker's own close button) — closeSplitPane() removes
+      // `.terminal-split-container` from the DOM, which detaches `divider`
+      // too, so `divider.parentElement` is null on the very next frame and
+      // every drag threw here until mouseup finally removed the listener.
+      if (!container) return;
+      const rect = container.getBoundingClientRect();
+      const rawPercent = ((clientX - rect.left) / rect.width) * 100;
+      const percent = window.CodemanSplitPane.clampDividerPercent(rawPercent);
+      wrap.style.flexBasis = `${percent}%`;
+      paneB.style.flexBasis = `${100 - percent}%`;
+      if (this.fitAddon) this.fitAddon.fit();
+      this._splitPane?.localFit();
+    };
+
+    const onMove = (e) => {
+      if (!dragging) return;
+      pendingClientX = e.clientX;
+      if (dragRaf) return;
+      dragRaf = requestAnimationFrame(() => {
+        dragRaf = null;
+        applyDragPercent(pendingClientX);
+      });
+    };
+
+    const onUp = (e) => {
+      dragging = false;
+      divider.classList.remove('dragging');
+      document.body.classList.remove('split-pane-resizing');
+      try {
+        divider.releasePointerCapture(e.pointerId);
+      } catch {
+        /* Already released (pointercancel/lostpointercapture beat us here). */
+      }
+      divider.removeEventListener('pointermove', onMove);
+      divider.removeEventListener('pointerup', onUp);
+      divider.removeEventListener('pointercancel', onUp);
+      if (dragRaf) {
+        cancelAnimationFrame(dragRaf);
+        dragRaf = null;
+        applyDragPercent(pendingClientX);
+      }
+      // Send the real PTY resize exactly once here, at drag end, for BOTH
+      // panes — never per-move (matching the codebase's established
+      // trailing-edge debounce convention, see throttledResize in
+      // terminal-ui.js) so a fast drag doesn't flood dozens of intermediate
+      // SIGWINCH/reflow states into scrollback or spawn a `tmux
+      // resize-window` child per frame.
+      this.sendResize?.(this.activeSessionId, { force: true })?.catch?.(() => {});
+      this._splitPane?.fit();
+    };
+
+    // Pointer events + setPointerCapture (mirrors tab-rail-resize.js) instead
+    // of mousedown/document-level mousemove: a plain mousedown drag selects
+    // the text under the cursor as it crosses both terminals, and pointer
+    // capture routes move/up straight to `divider` regardless of what's under
+    // the cursor mid-drag, so no document-level listener leak is possible if
+    // the pointer is released off-window. `body.split-pane-resizing` (mirrors
+    // `body.tab-rail-resizing`) locks the cursor/selection for the drag.
+    divider.addEventListener('pointerdown', (e) => {
+      if (e.button !== 0) return;
+      e.preventDefault();
+      dragging = true;
+      divider.classList.add('dragging');
+      document.body.classList.add('split-pane-resizing');
+      try {
+        divider.setPointerCapture(e.pointerId);
+      } catch {
+        /* Capture failed — the drag still works via the listeners below. */
+      }
+      divider.addEventListener('pointermove', onMove);
+      divider.addEventListener('pointerup', onUp);
+      divider.addEventListener('pointercancel', onUp);
+    });
+  },
+});
+
+const _originalOnSessionDeleted = CodemanApp.prototype._onSessionDeleted;
+CodemanApp.prototype._onSessionDeleted = function (data) {
+  if (this._splitSessionId === data.id) {
+    this.closeSplitPane();
+  } else if (this._splitPane && this.activeSessionId === data.id) {
+    // Pane A's session ended: promote Pane B by closing the split and
+    // selecting its session as the new (single) active pane. This is an
+    // app-driven selection, not the user clicking a tab, so it must not
+    // spend the promoted session's idle alert (see the Approvals Inbox
+    // acknowledgement rule in CLAUDE.md — only a human opening a session
+    // acknowledges it).
+    const promoted = this._splitSessionId;
+    this.closeSplitPane();
+    // Closing Pane A's own tab (closeSession(), app.js) adds data.id to
+    // _closingSessions BEFORE awaiting the delete, then owns the follow-up
+    // selection itself once the delete lands — same race _onSessionDeleted's
+    // own active-session handoff guards against (see its comment). Selecting
+    // here too would fight it for which tab wins.
+    if (promoted && !this._closingSessions.has(data.id)) {
+      this.selectSession(promoted, { auto: true });
+    }
+  }
+  return _originalOnSessionDeleted.call(this, data);
+};
+
+// I2: closes an active split BEFORE the primary pane rebinds to the same
+// session Pane B is showing (clicking Pane B's own session tab while split,
+// or any other selectSession() call that targets _splitSessionId). Without
+// this, Pane A rebinds to a session that Pane B's independent WebSocket is
+// still attached to — two live WebSockets to one session, each claiming PTY
+// dimensions via its own `{t:'z',...}` resize frame.
+const _originalSelectSession = CodemanApp.prototype.selectSession;
+CodemanApp.prototype.selectSession = function (sessionId, ...args) {
+  if (this._splitPane && this._splitSessionId === sessionId) {
+    this.closeSplitPane();
+  }
+  return _originalSelectSession.call(this, sessionId, ...args);
+};
@@ -1092,6 +1092,12 @@ Object.assign(CodemanApp.prototype, {
        if (this._localEchoOverlay?.hasPending) {
          this._localEchoOverlay.rerender();
        }
+        // Pane B (split view) has its own container and its own fit()/resize
+        // frame — this observer only ever measured Pane A's container, so
+        // without this call Pane B never learned about a window resize, an
+        // Alt+B sidebar toggle, or a tab-rail drag, and its PTY silently
+        // stayed at whatever size it was last dragged to.
+        this._splitPane?.fit();
      }, 300); // Trailing-edge: only fire after 300ms of no resize events
    };

@@ -5124,6 +5130,10 @@ Object.assign(CodemanApp.prototype, {
    // Update overlay font cache and re-render at new cell dimensions
    this._localEchoOverlay?.refreshFont();
    this._predictiveEcho?.refreshFont();
+    if (this._splitPane?.terminal) {
+      this._splitPane.terminal.options.fontSize = size;
+      this._splitPane.fitAddon?.fit();
+    }
  },

  /**
@@ -5149,6 +5159,10 @@ Object.assign(CodemanApp.prototype, {
    this.fitAddon?.fit();
    this._localEchoOverlay?.refreshFont();
    this._predictiveEcho?.refreshFont();
+    if (this._splitPane?.terminal) {
+      this._splitPane.terminal.options.fontFamily = resolved;
+      this._splitPane.fitAddon?.fit();
+    }
  },

  /**
@@ -5200,6 +5214,11 @@ Object.assign(CodemanApp.prototype, {
        /* pane not laid out yet — its own resize observer refits it */
      }
    }
+    if (this._splitPane?.terminal) {
+      this._splitPane.terminal.options.fontWeight = fontWeight;
+      this._splitPane.terminal.options.fontWeightBold = fontWeightBold;
+      this._splitPane.fitAddon?.fit();
+    }
  },

  loadFontSize() {
@@ -5420,6 +5439,13 @@ Object.assign(CodemanApp.prototype, {
        }
      }
    }
+    if (this._splitPane?.terminal) {
+      this._splitPane.terminal.options.minimumContrastRatio = minimumContrastRatio;
+      this._splitPane.terminal.options.theme = { ...theme };
+      try {
+        this._splitPane.terminal.refresh(0, this._splitPane.terminal.rows - 1);
+      } catch {}
+    }
  },
 });