From 24a92c8f3eb8dd58e7290486cdf3aeab955a0b86 Mon Sep 17 00:00:00 2001
From: timkjr <timkjr@k-lab.lan>
Date: Wed, 16 Sep 2026 20:24:23 -0500
Subject: [PATCH] fix(split-pane): refuse to split a session against itself
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Nothing stopped a stale picker click (opened before switching tabs) or
clicking Pane B's own session tab while split from landing on
openSplitPane(sessionId) with sessionId === activeSessionId, or from
selectSession() rebinding the primary pane onto the session Pane B was
already showing — either way, two live WebSockets to one session, each
independently claiming PTY dimensions via its own {t:'z',...} resize frame.
openSplitPane() now refuses early when the target is already the active
session, and a new selectSession() prototype patch (same top-level pattern
as the existing _onSessionDeleted patch) closes an active split BEFORE the
primary pane rebinds to the session Pane B holds.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 src/web/public/terminal-split.js | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/src/web/public/terminal-split.js b/src/web/public/terminal-split.js
index 96c378ce..473d2aa2 100644
--- a/src/web/public/terminal-split.js
+++ b/src/web/public/terminal-split.js
@@ -186,6 +186,12 @@ Object.assign(CodemanApp.prototype, {
   },
 
   openSplitPane(sessionId) {
+    // 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');
@@ -284,3 +290,17 @@ CodemanApp.prototype._onSessionDeleted = function (data) {
   }
   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);
+};