[BUG] Syntax Highlighting of C# disappears after a brief moment

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?

When I am working with the Claude app (on MacOS) in Code mode, and I expand to see the files and click a .cs file (C#), the file previewer displays the file with syntax coloring for a very brief moment, but then that blinks out of existence and goes into a clear black/white code display.

The expected behavior is that syntax coloring remains on the file, and is correct for each language that I open.

Notably, the issue does not happen with .js files (JavaScript). It happens too fast for me to see whether the file preview applies .js syntax coloring and then immediately regrets it when it realizes it is a .cs file. But yeah, this would be very handy to have resolved.

What Should Happen?

Expecting to see beautiful syntax coloring ALSO on my DotNet C# files, not only JavaScript when I click on a file in the files pane to preview it.

Error Messages/Logs

-

Steps to Reproduce

Preview a C# file (.cs)

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

1.11187.4 (584005) Claude for Mac

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

Other terminal is GhosTTY, but the issue relates to the desktop app. Just found it odd that Ghostty isn't on the list there :)