[BUG] [Desktop] Sidebar flattens fork hierarchy — nested forks shown as direct children of the root

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 new sidebar, forked sessions are shown as a tree under their parent session. However, only one level of nesting is rendered: forks of forks are displayed as siblings of their actual parent, directly under the root session.

This loses the information the tree is supposed to show. I use forks to split a task into subtasks (task → subtask → sub-subtask), and now I have to encode the hierarchy in session names manually.

What Should Happen?

Expected behavior
`
A
└ B
   └ C

Full fork tree with correct parent–child relations (ideally with collapse/expand per node).
`

Actual behavior
`
A
├ B
└ C

C is shown as a direct child of A, at the same level as B.
`

Error Messages/Logs

Steps to Reproduce

1. Create session A.
2. Fork A → session B (context menu → Fork, or from message).
3. Fork B → session C.
4. Look at the sidebar section containing these sessions.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

Claude Desktop build before 2026-10-15 update

Claude Code Version

2.37337.1

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Reproduced in the Claude Desktop app (Code tab), not in a terminal — Terminal/Shell field is not applicable.
Session names encode the real hierarchy (RM18734 > 18767 > 19142), which makes the flattening visible in the attached screenshot.