[BUG]

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?

A black vertical bar appears between the left sidebar and the prompt/chat area, partially obscuring the prompt input. The issue affects both the Chat and Claude Code interfaces and persists across all open threads once triggered.

Platform

• macOS (Intel-based Mac, not Apple Silicon) v - 13.7.8

<img width="1198" height="765" alt="Image" src="https://github.com/user-attachments/assets/1e1a2824-be97-4b86-a5ca-a0cb5fc72f94" />

• Claude Desktop (latest version — updated twice in the past two days, issue persists through both updates)

What Should Happen?

The layout should remain stable and the prompt area fully visible at all window sizes.

Actual Behavior
A black bar intrudes between the sidebar and prompt area after resizing, growing larger with continued resizing and affecting all threads globally.

<img width="1198" height="765" alt="Image" src="https://github.com/user-attachments/assets/b6fc82ec-7b30-4056-b8d9-d03ca6195bae" />

Error Messages/Logs

Steps to Reproduce

1. Launch Claude Desktop
2. Open any chat thread
3. Resize the window (width or height) several times
4. After a few resizes, a black bar begins to appear between the sidebar and prompt area — initially small, then grows larger
5. Switching to other chat threads causes the bar to appear there as well

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

1.4758.0

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Key Observations

• The bar does not appear immediately — it builds up incrementally after multiple resizes
• Making the window taller/larger keeps the bar clear; it appears when vertical/horizontal space is more constrained
• Once triggered, it spreads to all chat threads (shared window state)
• Clearing Cache, GPUCache, and window-state.json temporarily resolves it, but resizing re-triggers the issue
• Full app updates have not resolved it

Workaround
Keeping the window maximized or very tall or long prevents the bar from appearing, but this is not viable as it blocks access to other applications needed alongside Claude.