[BUG] Local dev-server preview opens twice — a second, chrome-less panel overlays the chat and cannot be dismissed

Environment

• Claude Desktop 1.24012.9 (macOS, Apple Silicon), with Claude Code 2.1.219 embedded

What happens

Opening a local dev-server URL from inside the app renders the page twice:

1. The normal Browser pane on the right, with tab strip and URL bar.
2. A second, borderless panel on the left that overlays the conversation.

Steps to reproduce

1. Run a local dev server (Vite on port 5190).
2. In a Claude Code chat, either click a localhost link in a message, or click Open on the inline preview card.
3. Both surfaces appear from that single click.

The second panel

• No title bar, no close button, no URL bar; hovering reveals no controls.
• <kbd>Esc</kbd> does not dismiss it.
• It covers the chat transcript and the input box, so the conversation can't be read or used while it is up.

Diagnostics

• macOS accessibility reports a single application window, so this is not a stray OS window.
• DevTools on the Claude Code renderer shows no iframe/webview for either surface — both appear to be native views layered over the window, so nothing in the page can close or hide it.

What does NOT reproduce it

An agent-side preview_start renders a small inline preview card in the transcript and opens no panel at all — no duplication, and the chat stays fully visible.