[BUG] Browser pane detached to separate window is slow/visually corrupted — regression in 1.49585.0

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

Detaching the in-app Browser pane into a separate OS window causes visible frame corruption (pixelated/static overlay across the whole page) and consistently slow, choppy frame delivery. The docked pane (side panel) renders correctly.

### What Should Happen?

The detached window should render smoothly and correctly, the same as the docked pane or a normal browser tab on the same page.

### Error Messages/Logs

```shell
None found. Checked dmesg for GPU/DRM errors, journalctl for claude-desktop crash/hang/timeout entries, and the Crashpad directory for dump files — all clean. The bug shows up as visual corruption, not a logged error or crash.
```

### Steps to Reproduce

1- Open the Browser pane docked in the side panel and navigate to any local or web page.
2- Pop it out into a separate window.
3- Observe pixelated/static-like corruption over the page content and slow, choppy frame updates when scrolling or when the page has motion (e.g. autoplay video).

### Claude Model

Sonnet (default)

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

1.46388.2

### Claude Code Version

2.1.260

### Platform

Anthropic API

### Operating System

Ubuntu/Debian Linux

### Terminal/Shell

Other

### Additional Information

Ruled out local causes before concluding this is a regression: cleared Claude Desktop's cache (Cache, Code Cache, GPUCache — ~480MB) and did a full device reboot, neither fixed the detached-window issue (though the cache clear did fix a separate, unrelated slowdown in the docked pane caused by a prior crash + force-kill). A plain Chrome tab on the same localhost URL renders smoothly, confirming it isn't the page/dev-server content. Downgrading to 1.46388.2 and pinning with apt-mark hold fully resolved the detached-window corruption/lag, confirming the regression was introduced somewhere between 1.46388.2 and 1.49585.0. Screenshot of the corruption available on request.

Start by reproducing the detached Browser pane on Ubuntu/Debian with Claude Code 2.1.260, comparing it with the docked pane and a plain Chrome tab. Compare the regression version 1.49585.0 with the last working 1.46388.2, then narrow the detached-window rendering path. Done means no pixelation and smooth frame delivery during scrolling and autoplay video.

索引モデルが issue の本文から書いたものです。