Why

Dragging a session row between the sidebar's Pinned and Sessions sections silently does nothing (operator video, 2026-06-10): each section owns a private dnd-kit DndContext scoped to in-section reorder from the hover grab handle, and the row body's native HTML5 drag only targets the composer (inline session refs). So the drag gesture exists — the row ghost follows the pointer — but the drop is a no-op, and pin/unpin requires the context menu or shift-click. Claude Desktop supports drag-to-pin/unpin; Hermes should match.

What changed

• apps/desktop/src/app/chat/composer/inline-refs.ts — SessionDragPayload gains pinId (durable lineage-root id, what the pin store keys on) and pinned. writeSessionDrag additionally sets a marker MIME application/x-hermes-session-pinned for pinned rows — dragover events expose only types (payload data is sealed until drop), so pin-state must ride as a type for zones to filter mid-drag. New dragSessionIsPinned(); readSessionDrag normalizes pinId (falls back to the live id for old payloads).
• apps/desktop/src/app/chat/sidebar/session-row.tsx — the row's native dragstart payload now includes pinId: sessionPinId(session) and pinned: isPinned.
• apps/desktop/src/app/chat/sidebar/use-session-drop-zone.ts (new) — useSessionDropZone({ acceptPinned, onDropSession }): native drop target with an enter/leave depth counter (mirrors use-file-drop-zone), dropEffect = 'copy' to match the drag's effectAllowed, and a guard so stray leave events from unaccepted drags can't drive the depth negative. A zone only preventDefaults drags it would act on, so no-op drops never advertise.
• apps/desktop/src/app/chat/sidebar/index.tsx — ChatSidebar wires two zones: Pinned accepts unpinned-row drags → pinSession(pinId); Sessions accepts pinned-row drags → unpinSession(pinId). Both auto-open the target section on drop. SidebarSessionsSection spreads the handlers on its SidebarGroup root (header included — drops work while collapsed or empty) and highlights while targeted (bg-(--ui-control-hover-background) + inset ring).
• apps/desktop/src/app/chat/sidebar/use-session-drop-zone.test.tsx (new) — 9 tests: payload round-trip incl. pin metadata + legacy-payload fallback, accept/ignore by pin-state, non-session drags ignored, nested-children depth counting, stray-leave wedge guard.

Existing behaviors unchanged: grab-handle reorder within each section (dnd-kit), drag-into-composer (the same native drag), shift-click and context-menu pin.

How to review

1. use-session-drop-zone.ts first — the accept rule (dragHasSession && dragSessionIsPinned === acceptPinned) is the heart of the change.
2. inline-refs.ts — note the marker-MIME rationale (types-only visibility during dragover) and that readSessionDrag stays backward-compatible.
3. index.tsx — the two useSessionDropZone calls and the SidebarGroup spread; confirm the highlight classes only attach when dropActive.
4. session-row.tsx — two added payload fields; the dnd-kit handle still cancels the native drag via [data-reorder-handle], so the two DnD systems stay separated.

Evidence

• Live render verification in real Chromium against the vite dev renderer (sidebar mounted with seeded session/layout stores reproducing the operator's video scenario):
  • dragging the pinned "KO Plex Overhaul Project feat: add HERMES_LLM_BASE_URL fallback for MeshBoard stream-tap proxy #23" row over Sessions lit the section highlight; drop removed it from $pinnedSessionIds (['s-pin-1'] → []) and the row re-rendered at the top of Sessions while Pinned fell back to its empty-state hint;
  • dragging "What model are you?…" over the now-empty Pinned section lit its highlight; drop landed $pinnedSessionIds = ['s-2'] and the row rendered under Pinned;
  • DataTransfer.types during the pinned-row drag: ["application/x-hermes-session","application/x-hermes-session-pinned"]; unpinned-row drag carried only the session MIME.
• npx vitest run --environment jsdom src/app/chat/sidebar/use-session-drop-zone.test.tsx: 9/9 pass.
• Full renderer suite failure set is byte-identical to baseline (see Verification) — this diff adds 0 failures.

Verification

• npm run type-check (tsc -b): pass.
• npx eslint on all five touched files: pass, no warnings.
• npx vitest run --environment jsdom full renderer suite at origin/main (559b472) + this diff: 485 tests, 476 passed / 9 failed; the sorted FAIL line set is byte-identical (diff-verified) to the baseline run with this change stashed — the known baseline-red 9 (prompt-actions, gateway-boot, model-settings, toolset-config, streaming, pane-shell) plus the .cjs/node-pty file-level entries vitest always flags (those run under npm run test:desktop:platforms).
• npm run build (tsc -b + production vite build): pass.

Risks / gaps

• Dropping a session that lives outside the recents slice (e.g. a messaging-platform row) pins its id but the Pinned section can't resolve it for rendering — identical behavior to the existing context-menu Pin on those rows, so no regression; tracked on the MeshBoard task hermes-desktop-drag-sessions-between-sessions-and as a known pre-existing limitation.
• Drag-to-pin appends to the end of the pinned list rather than inserting at the pointer position; in-section grab-handle reorder covers ordering. Accepted scope cut to keep the native-DnD path simple — noted on the same MeshBoard task for a possible polish follow-up.
• The pinned-state marker rides the drag from dragstart, so a row pinned/unpinned mid-drag by another surface could stale the flag; the drop handlers are idempotent against the store (insertUniqueId dedups, unpin filters), so the worst case is a no-op drop. Non-actionable.
• Upstream port: NousResearch/hermes-agent carries the same inline-refs.ts drag plumbing and sidebar section structure; the paired upstream PR is opened from an upstream/main-based branch per the dual-PR mandate (link tracked on the MeshBoard task).

Collaborators

• @OmarB97 (operator — reported the issue with a screen recording)
• Claude Fable 5 (ko-mac.claude — implementation, tests, verification)