[BUG] Resume picker shows `>` group indicator on forked sessions but arrow keys do not expand/collapse (2.1.133)

Preflight Checklist

• [x] I have searched existing issues — closest match is #19451 (closed as stale but unfixed)
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

The resume picker's grouped-fork UI is half-broken. The > chevron indicator that marks a collapsible group of forked sessions renders correctly next to forked entries, but the documented → / ← keys to expand and collapse the group have no effect. The forked child session is therefore unreachable through the picker.

This is closely related to #19451 (closed as stale 2026-03-13) and the bisection in its comments — toggles working on 2.1.29 and missing from 2.1.31 onward, possibly tied to the session-index rewrite in 2.1.30. The regression is still present in 2.1.133.

Concrete repro from my session

• Session A (original): d5f8f352-cc0f-4291-9296-f6ff6e05d836.jsonl — 249 lines, custom name set via Ctrl+R (LLM-coding-workflowy-design), AI-title also present.
• Session B (forked from A): 986ea194-bd83-4c12-aa42-3a8c1f3234b1.jsonl — 197 lines, no custom title. Diverges from A at the same promptId with a different parentUuid.
• Both files are present on disk under ~/.claude/projects/-home-james/.

In the resume picker:

• Session B (the fork, currently HEAD) appears in the flat list.
• Session A appears with a leading > chevron, labelled with its custom name LLM-coding-workflowy-design.
• Pressing → while that entry is the highlighted row does nothing. Pressing ← likewise. The group never expands. The fork descendant is not reachable from the picker.
• The footer shows Ctrl+A to show all projects · Ctrl+B to only show current branch · Space to preview · Ctrl+R to rename · Type to search · Esc to cancel — →/← are not documented in the footer at all, despite being documented at https://code.claude.com/docs/en/common-workflows.
• Ctrl+B ("only show current branch" / toggle off) does not change which entries are visible.

This is distinct from the in-file parentUuid branch case in #19451 — here the fork is in a separate session file, so the picker clearly does have enough information to group them (it renders the > chevron). The expansion keybinding is the only missing piece.

What Should Happen

• Pressing → on a >-marked group entry expands it to show its forked children.
• Pressing ← collapses it again.
• The footer should list →/← alongside the other documented keybindings.

Workaround

claude --resume <session-id> resumes any session by ID regardless of picker visibility. Locating the ID requires ls ~/.claude/projects/<project>/ plus inspecting the JSONL contents — there is no UI affordance for getting an ID out of the picker (related: #54389).

Environment

• Claude Code version: 2.1.133
• OS: Linux (Arch, kernel 6.18.9-arch1-2)
• Shell: bash
• Terminal: Konsole

References

• #19451 — same symptom, closed as stale, never fixed
• #48270 — related: --resume picks stale branches on forked sessions, compounding
• #54389 — feature request: show session IDs in picker (the workaround above would be much easier with this)
• #56498 — /branch docs omit the branched session ID