[BUG] Web UI diff view only shows files changed within the current session, not the full branch diff vs. base

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?

In the Claude Code web UI, the diff panel shows main ← claude/openspec-setup-e2jv9 but only displays files changed within the current session, not all files that differ between the branch and main. In this case, 406 files existed on the branch from a prior session, but the diff only showed the 1 file (.gitignore) added in the current session. GitHub itself correctly shows the full branch diff. Expected: the diff view should reflect the complete delta between the feature branch and the base branch.
Also, it seems this may be hapenning because the original github repo was really empty (ie, created without main)

What Should Happen?

Show the files that are indeed committed.

Error Messages/Logs

Steps to Reproduce

1. create new project in github, without main branch;
2. create a new session in claude code web;
3. do some action that creates files in the project;
4. ask them to be committed;
5. confirm they appear in github on the non-main branch;
6. the UI will show none of the files as needing to be committed;

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

web

Platform

Anthropic API

Operating System

Other

Terminal/Shell

Other

Additional Information

_No response_