QT-parity docking: auto-hide sidebar UI — collapsed edge-rail + runtime reveal + pin affordance

Context

Epic #13158 (QT-parity docking polish), the auto-hide/pin gap. The model seam is DELIVERED — #13254 (PR #13256, merged) landed the persisted autoHidden item field, the setItemAutoHidden semantic operation, and the pin/auto-hide coherence (setItemPinned(…, true) clears autoHidden). This leaf is the UI consumer of that seam: the visible collapsed-to-sidebar affordance QT docking treats as table stakes.

The Problem

DockZoneModel can now persist autoHidden: true, but nothing renders it — DockLayoutAdapter projects every item as a full split/tab child regardless of autoHidden, and there's no reveal-on-hover/click or pin control. An auto-hidden item is currently invisible to the user.

The Fix (UI consumer of the #13254 model seam)

1. Collapsed edge-rail render — DockLayoutAdapter projects an autoHidden: true item as a thin edge rail (icon/title tab on the owning edge zone), not a full pane.
2. Runtime-only reveal — hovering/clicking the rail overlays the item's content transiently. The reveal/hover/open geometry is runtime-only, never persisted (JSON-first guardrail — HarnessDockZoneModel.md §Serializable vs Runtime).
3. Pin affordance — a pin control toggles setItemPinned(itemId, true) — which, per the merged model coherence, clears autoHidden and restores the item to a full pane.

Contract question (claimer settles at claim time, per #13158)

• The rail projection mode — a new DockLayoutAdapter branch for autoHidden items vs an edge-zone decoration.
• The reveal mechanism — a runtime-only overlay riding the existing reactive DockPreview-style pattern (visual-only, no parallel pointer system), or a container collapse.
• The pin-affordance location + how it rides the existing preview→operation descriptor path.

Binding guardrails (ADR 0020 §4)

• JSON-first — no DOMRect/hover/reveal/open geometry in any serialized layout; reveal state stays runtime-only and converts to setItemPinned/setItemAutoHidden on commit.
• No parallel drag/pointer system — reuse the existing overlay/operation pipeline.
• No public performance claims before the Harness Endurance Benchmark (#13032).

Contract Ledger Matrix

Acceptance Criteria

• An autoHidden: true item renders as a collapsed edge rail (not a full pane).
• Hovering/clicking the rail reveals content transiently; the reveal state is never persisted (round-trip carries no hover/reveal geometry).
• A pin affordance toggles setItemPinned(…, true), clearing autoHidden and restoring the full pane.
• Focused unit coverage on the projection + the no-persisted-reveal guardrail; vega's examples/dashboard/dock/ (#13247/#13278) gains a Slice demonstrating auto-hide (coordinate the slice boundary).

Dependencies

• #13254 / PR #13256 (MERGED) — the model seam (autoHidden + setItemAutoHidden + pin coherence) this leaf consumes.

Out of Scope

• Named perspectives + grouped tab-drag (sibling #13158 leaves).
• Any change to the persisted dockZone.v1 model (the seam is complete; this is render + runtime only).

Related

• Epic #13158 (QT-parity docking polish). Model seam: #13254 (merged). Verification surface: #13247 / #13278 (@neo-opus-vega's standalone dock example). Substrate: src/dashboard/DockLayoutAdapter.mjs, src/dashboard/DockZoneModel.mjs, apps/agentos/view/DockPreview.mjs. Guardrails: ADR 0020.

Evidence: L2 (unit on the rail projection + the no-persisted-reveal guardrail) → L3 (visual verification via vega's #13247/#13278 example) required for the UI affordance.

Authored by Claude Opus 4.8 (Claude Code), @neo-opus-grace (Grace).