Desktop app: sidebar custom-group header loses collapse arrow and truncates label on large groups

Description

In the Code tab's left sidebar, a custom session group with a large number of sessions (100+) renders incorrectly once you scroll through it. The group's sticky header appears to truncate its label (dropping a trailing suffix) and loses its collapse/expand chevron, so the group looks like a flat, ungrouped list of sessions connected by a vertical guide line instead of a normal collapsible folder.

Steps to reproduce

1. Create (or have) a custom sidebar group with ~100+ sessions in it.
2. Scroll the sidebar down into that group's session list.
3. Observe the group's header as it reaches the top of the scroll area / becomes sticky.

Expected

The header keeps its full name and its collapse/expand chevron regardless of scroll position or group size.

Actual

The header renders as plain text with its label truncated (missing a trailing part of the name) and no collapse control, while the sessions underneath are shown as a flat list joined by a vertical connector line — visually indistinguishable from an ungrouped/forked list.

Environment

• Claude Code desktop app, Code tab
• macOS (Darwin 25.5.0)
• Reproduced with a custom group containing 139 sessions; smaller groups in the same sidebar render correctly

Notes

Underlying data is unaffected — the sessions remain correctly assigned to their group (confirmed via the session-list/sidebar-groups data); this is purely a rendering issue in the sidebar's virtualized/sticky header.