[BUG] Claude Chrome extension renders black screen with no tool access in Vivaldi browser (Linux)

Preflight Checklist

• [x] I have searched existing issues and this specific behavior hasn't been reported
• [x] I can reproduce the issue

Description

When using the Claude Chrome extension in Vivaldi browser on Linux, opening the extension tab renders a completely black background with no UI content and no access to the Claude tool. The tab opens but is non-functional — there are no controls, no chat interface, and no error messages visible to the user.

This appears to be a distinct issue from the native messaging host detection problems reported in #14370, #14391, and #14536. In those issues the extension installs and the panel renders, but Claude Code CLI cannot detect or communicate with it. Here, the extension UI itself fails to render inside Vivaldi.

Environment

| Component | Version |
|---|---|
| OS | Arch Linux (rolling) |
| Kernel | 6.19.6-arch1-1 (x86_64) |
| Vivaldi | 7.8.3925.81 stable |
| Claude Chrome Extension | 1.0.59 (fcoeoabgfenejglbffodgkkbkcdhcgfn) |
| Claude Code CLI | 2.1.56 |

Steps to Reproduce

1. Install the Claude Chrome extension from the Chrome Web Store in Vivaldi
2. Extension installs successfully and appears in the toolbar
3. Click the extension icon or open the extension's tab
4. Observe a blank tab with a black background — no UI renders

Expected Behavior

The Claude extension tab should render its full interface (chat panel, controls, connection status) the same way it does in Google Chrome.

Actual Behavior

A tab opens with a solid black background. No Claude UI elements are visible. There is no way to interact with the tool. No error messages or loading indicators are displayed.

Suspected Cause

Vivaldi has known differences in how it implements certain Chrome extension APIs compared to other Chromium-based browsers (Brave, Edge, Arc). Specifically:

• Vivaldi's sidebar/panel implementation differs from Chrome's side_panel API — Vivaldi uses its own Web Panels system and may not fully support chrome.sidePanel or the extension's expected rendering context
• Extension page rendering context: Vivaldi may handle the background/popup/tab rendering lifecycle differently, causing the extension's UI entry point to fail silently
• CSP or iframe sandboxing differences: Vivaldi's security model for extension-hosted pages may block the resources the Claude extension needs to render

This is likely a separate root cause from the native messaging host path detection issue (#14391) and the browser selection issue (#14536), since the problem here is at the extension UI rendering level rather than the CLI-to-extension communication layer.

Related Issues

• #14370 — Detect Claude Chrome extension in other Chromium-based browsers
• #14391 — Native messaging host not installed for Chromium/Brave browsers (Linux)
• #14536 — Allow browser selection instead of opening default browser
• #18075 — Add env var for custom Chromium browser path