fix(sidebar): empty groups show only their header
Empty workspace groups now show only their header at rest. The header's + stays reachable on hover, keyboard focus and accessibility actions; dragging a workspace into an empty group highlights that header. Full layout and fast fold no longer insert “No workspaces · drag one here”.
Leo, 2026-11-01: “what do you feel about this placeholder situation? idk doesn't that plceholder text seem a bit wack?”
The gallery compares three native versions: the previous placeholder row, header-only (chosen), and an always-visible header +. Each includes rest, hover and drop states in light and dark at 2x. Header-only avoids an extra row while keeping the existing creation and drop actions.
Validation: focused tests cover header-only geometry, fold/full-layout agreement, dropping on the empty header, transient drag highlight, and hover + creation. Test-only red reference: 86fa5cfa7461; implementation head follows it. Fleet compile/test evidence follows. Static prepush gate passes; the lane has no Swift compiler. Gallery artifacts are follow-up visual evidence, not a merge gate.
Risk: limited to projecting an empty group and its drop highlight. Group persistence and last-workspace close behavior are handled separately in #30292. No new settings or defaults.
Design: leo-workspace design/sidebar.md; recorded in plans/cmux-next/sidebar-owner.md. Replaces the old placeholder treatment, preserving its create/drop actions. Prior art checked: #7061, #23182, #24581, #25669 and #21835; none duplicates this presentation fix.
Changelog: Empty sidebar groups keep a compact header with creation available on hover.
Agent session: agent session
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Summary by cubic
Empty sidebar groups now show only their header; the placeholder row is removed from both full layout and fast fold. The header’s + remains available on hover, keyboard focus, and accessibility actions, and highlights when a workspace is dragged into the group.