[BUG] New tab button opens split instead of tab in same group

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

<img width="271" height="123" alt="Image" src="https://github.com/user-attachments/assets/ea67aa0d-59b0-407b-bfa1-24a49b73a4b4" />

<img width="1371" height="816" alt="Image" src="https://github.com/user-attachments/assets/67ec5794-8ed6-45e5-9d6b-88700ecfba74" />

When Claude Code is open as a panel in VS Code with a certain number of conversation tabs open, clicking the "+" button (New Conversation) causes a new Claude Code panel to open in a new split editor column to the right, instead of adding a new conversation tab within the existing editor group. This behavior is reproducible and has been observed multiple times.

Environment

OS: macOS (darwin arm64)
VS Code version: 1.120
Claude Code extension version: 2.1.143
Additional context

What Should Happen?

The new conversation should open as a new tab within the same editor group, not in a new split column.

Error Messages/Logs

The editor splits horizontally and the new conversation opens in a separate editor column on the right. Dragging the new tab back to the original editor group works, but the next time "+" is clicked, the split happens again. The bug is consistently reproducible once it starts occurring.

Steps to Reproduce

Open Claude Code in VS Code (single editor column, full width)
Click the "+" (New Conversation) button at the right end of the Claude Code tab bar
Observe: a new split editor column appears on the right with a new "Untitled" Claude Code tab

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.143

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

Last time i had 8 tabs opened when this bug happened.