#24513 cmux NEXT sidebar: Chrome's group line shape on the header's rail (cx-qno.27.44, cx-qno.27.45, cx-qno.27.46)

A sidebar group's line now uses Chrome's thin, flat-ended shape and the header band's color. Members keep the ungrouped rows' left edge. This PR covers only group-line chrome: 424 insertions and 46 deletions (470 changed lines).

Leo lifted the sidebar freeze for this contained visual-only change on 2026-10-23. The PR has no model, persistence, or data-flow changes; the risk is limited to sidebar geometry, color, and animation layering for group headers and rails.
Leo's earlier hold instruction, retained here for the record, was:

Held sidebar PRs keep their hold label and go into dogfood builds only.

The owner has since lifted that hold for this small visual-only change.

Leo-ask: ask-5b16be6ee9
Leo-ask: ask-40133bf821
Leo-ask: ask-7e10ebd553

Supersedes #21999. Tracking: cx-qno.27.44, cx-qno.27.45, cx-qno.27.46.

Behavior and before/after

The line is 2 pt wide at every density, with square ends and a 6 pt cut below the header. Under the solid band it uses the band's exact rest color. Nested rails retain their existing depth opacity.

Before/after gallery (base before at left; current group-line variants after):

The native AAGroupLineGalleryTests gallery contains three Debug Settings variants (sidebar.groupLineLayout), each in light and dark with selected members, nested groups, and a collapsed group:

Version	Behavior
edge (selected default)	Line flush with the header's left edge, separated by the cut.
joined	Line meets the band's square bottom-left corner without a cut; other header styles retain the cut.
inside	Header reaches further left so the line sits inside its corner, clamped at the list edge.

Pick: edge. It keeps the header and rail aligned with the current sidebar geometry and adds no second member indent. The other two remain available for dogfood comparison.

Design reference: cmux-design-references/apps/cmux/live/sidebar-codex-group-rows-vs-chrome.png; also the Claude sidebar nesting reference. Leo's screenshots stay private. In words, the requested treatment is a thin colored group rail with flat ends, a small cut under the header, and no extra indent inside the member rows.

The repair keeps inside rails inset when a parent has only nested groups, with a regression fixture covering that geometry.

Observed / Not run

Candidate head: 109e3baaf1734f227cc87748c14eef8851d02635.

• Ownership claimed by Codex cx4-sb24513 on cmux16.
• Isolated sparse worktree created from origin/feat-cmux-next; no shared checkout edits or Air Blue builds.
• Independent Codex gpt-6.1-sol xhigh review: PASS at this exact head; the review found and the repair fixed the parent-only nested-group corner-inset defect.
• Dogfood build completed on the repair head: e67d7b21ea1702c620918a50, tag pr-24513-group-line-codex-v2, artifact SHA256 48943a5832db1c7afa4e82546730bd31e18c38a9a81b3ee085c7fcd42f6c6fa0, pinned to this head.
• Focused fleet suite step 393b6c0d543fd7c42da37ff4 is pinned to this exact head and remained queued during the bounded wait; the passing repair-head dogfood build above is the compile receipt.
• The before/after images above are public demo-fixture gallery media; no private screenshots are included. A media-tour label is present so the current head also receives the repository's real capture workflow.

Changelog

• Fixed: Sidebar group lines use a thin, flat-ended rail in the header band's color, with aligned, joined, and inside-corner dogfood variants.

Agent session: cmux16