Why

Drag-to-pin (#145) shipped with three documented gaps, all called out in its Risks/gaps and on the MeshBoard task hermes-desktop-drag-sessions-between-sessions-and:

1. Pinned messaging rows silently vanish. sessionByAnyId indexes only cron + recents, so pinning a messaging-platform row (context menu, shift-click, or the new drag) stores an id the Pinned section can never resolve — the pin takes effect in the store and renders nothing. This predates feat(desktop): drag sessions between Pinned and Sessions to pin/unpin #145 (the menu had the same bug) but drag-to-pin made it much easier to hit.
2. Drops append instead of inserting. Drag-to-pin always appended to the end of the pinned list; drag-to-unpin always surfaced the row at the top of Sessions. Claude Desktop inserts at the pointer.
3. The gesture is undiscoverable. The empty-Pinned hint only mentioned shift-click.

What changed

• apps/desktop/src/app/chat/sidebar/index.tsx
  • sessionByAnyId now also indexes $messagingSessions (recents still win id collisions), so messaging pins resolve and render in Pinned. Side benefit: server FTS hits for loaded messaging sessions resolve to the real row instead of a synthesized placeholder.
  • messagingGroups filters pinnedRealIdSet so pinning MOVES the row out of its platform section (same semantics as recents) instead of duplicating it; a platform whose every row is pinned drops its section.
  • The two drop handlers are positional. Pin: translate the anchor row under the pointer into an index in the RAW pinned-id store by indexOf of the anchor's durable pin id — rendered rows can be a subset of stored ids, so rendered position is not trusted. Unpin: splice the dropped id into the saved flat-list order before/after the anchor; header drops, fresh-row anchors, and grouped/ALL-profiles views keep the previous behavior (recency reconcile surfaces the row at the top). The handler block also moved below the memos it reads so declaration order matches data flow.
• apps/desktop/src/app/chat/sidebar/use-session-drop-zone.ts — onDropSession now receives the drop event; new exported sessionDropAnchor(event) resolves the [data-session-id] row under the pointer with top-half/bottom-half = before/after semantics.
• apps/desktop/src/app/chat/sidebar/session-row.tsx — rows carry data-session-id (used by the anchor lookup; also handy for tests).
• apps/desktop/src/i18n/{en,zh,zh-hant,ja}.ts — empty-Pinned hint becomes "Drag a chat here, or shift-click to pin" (each locale keeps its existing pin/chat terminology).
• apps/desktop/src/app/chat/sidebar/use-session-drop-zone.test.tsx — 4 new sessionDropAnchor tests (before/after midpoint, nested children, null off-row); drop tests updated for the (payload, event) signature.

How to review

1. use-session-drop-zone.ts — sessionDropAnchor and the event pass-through are the whole mechanism.
2. index.tsx drop handlers — the pin-index translation comment explains why anchor → raw-store index goes through the durable pin id; the unpin path's guards (flat-list only, anchor must exist in the saved order) fall back to pre-positional behavior rather than guessing.
3. index.tsx messaging changes — one added list in the index memo + one continue guard in the platform-split loop; check the dep arrays picked up messagingSessions / pinnedRealIdSet.
4. Locale files — copy-only.

Evidence

Live verification in real Chromium (vite dev renderer, seeded stores; same harness as #145):

• Positional pin: dropping "Recent Two" on the bottom half of "Pinned Alpha" landed $pinnedSessionIds = ["s-pin-1","s-2","s-pin-2"] — between the two existing pins, rendered in that order.
• Positional unpin: dropping "Pinned Bravo" on the top half of "Recent Three" produced saved order ["s-1","s-pin-2","s-3","s-4"] and Sessions rendered One → Bravo → Three → Four.
• Messaging pin: dragging "Discord thread B" onto Pinned gave pinnedIds=[...,"m-2"], the row rendered under Pinned, and the Discord section listed only m-1 — per-section row audit: Pinned => [s-pin-1,s-2,m-2] || Sessions => [s-1,s-pin-2,s-3,s-4] || Discord => [m-1].
• Empty-Pinned hint renders "Drag a chat here, or shift-click to pin".
• npx vitest run --environment jsdom src/app/chat/sidebar/use-session-drop-zone.test.tsx: 13/13 pass.

Verification

• npm run type-check (tsc -b): pass.
• npx eslint on all eight touched files: pass, no warnings.
• Full renderer suite (npx vitest run --environment jsdom) at origin/main (410c71f) + this diff: no new failures — the sorted FAIL set equals the documented baseline minus the two use-prompt-actions file-attach failures that fix(desktop): restore file.attach client upload path (Phase 2b companion to #142) #144 fixed on main (diff-verified against the recorded baseline set).
• npm run build (tsc -b + production vite build): pass.

Risks / gaps

• A platform section disappears entirely when ALL its loaded rows are pinned; the rows live in Pinned and unpinning restores the section. Intentional (mirrors recents), noted here for reviewers — non-actionable.
• Per-platform "load more" counts treat pinned rows as not-loaded (loaded count shrinks by the pinned rows), so a step can over-report by the number of pinned rows for that platform. Worst case is an off-by-a-few label on the load-more affordance; accepted, noted on the MeshBoard task with the messaging-pin item.
• Positional unpin into a grouped/ALL-profiles view intentionally degrades to the recency-reconcile (top) placement — those views derive their own order, so a saved-order splice would be invisible anyway. Accepted scope.
• Upstream port: these follow-ups are cherry-picked onto the open upstream PR branch (feat(desktop): refine sidebar session drag reordering NousResearch/hermes-agent#43661) so the port stays complete; tracked on the same MeshBoard task.

Collaborators

• @OmarB97 (operator — requested the follow-ups)
• Claude Fable 5 (ko-mac.claude — implementation, tests, verification)