fix(desktop): deduplicate sidebar rows by compression lineage in mergeSessionPage (#43487)

When auto-compression rotates the session tip (old #4 → new #5), the
incoming page carries the new tip but the previous list still holds the
old one. The old tip's id differs from the new tip's id, so the existing
id-only dedup in mergeSessionPage() preserves both as separate sidebar
rows.

Add lineage-level dedup: build a set of incoming lineage keys
(`_lineage_root_id ?? id`) and filter survivors whose lineage key
matches any incoming row. This mirrors the existing sessionPinId()
logic used for pin stability.

Fixes #43483

apps/desktop/src/store/session.test.ts

@@ -133,13 +133,52 @@ describe('mergeSessionPage', () => {
   it('keeps a pinned session matched by its lineage root after compression', () => {
     // The pin is stored on the lineage-root id, but the loaded row surfaces
     // under its live compression tip. Matching on _lineage_root_id keeps it.
-    const previous = [session({ id: 'tip', _lineage_root_id: 'root' })]
-    const incoming = [session({ id: 'other' })]
+    const previous = [session({ id: 'tip', _lineage_root_id: 'root' })] as SessionInfo[]
+    const incoming = [session({ id: 'other' })] as SessionInfo[]
 
     const merged = mergeSessionPage(previous, incoming, ['root'])
 
     expect(merged.map(s => s.id)).toEqual(['tip', 'other'])
   })
+
+  it('evicts an old compression tip when the incoming page has the new tip from the same lineage', () => {
+    // Repro of #43483: after auto-compression rotates the tip (#4 → #5),
+    // the sidebar showed both the old tip and the new tip as separate rows.
+    // The old tip must be evicted because its lineage key matches the incoming
+    // new tip's lineage key.
+    const previous = [
+      session({ id: 'tip-4', _lineage_root_id: 'root' }),
+      session({ id: 'other' }),
+    ] as SessionInfo[]
+    const incoming = [
+      session({ id: 'tip-5', _lineage_root_id: 'root' }),
+    ] as SessionInfo[]
+
+    // 'tip-4' is in the keep set (e.g. it was the active/working session),
+    // but should still be evicted because the incoming page carries the same
+    // lineage under a new tip id.
+    const merged = mergeSessionPage(previous, incoming, ['tip-4'])
+
+    expect(merged.map(s => s.id)).toEqual(['tip-5'])
+    // The new tip comes from the server payload.
+    expect(merged.find(s => s.id === 'tip-5')?._lineage_root_id).toBe('root')
+  })
+
+  it('preserves an unrelated pinned session even when lineage dedup is active', () => {
+    // Regression guard: lineage dedup must not accidentally evict sessions
+    // from a different lineage that happen to be in the keep set.
+    const previous = [
+      session({ id: 'a-old', _lineage_root_id: 'lineage-a' }),
+      session({ id: 'b', _lineage_root_id: 'lineage-b' }),
+    ] as SessionInfo[]
+    const incoming = [
+      session({ id: 'a-new', _lineage_root_id: 'lineage-a' }),
+    ] as SessionInfo[]
+
+    const merged = mergeSessionPage(previous, incoming, ['b'])
+
+    expect(merged.map(s => s.id)).toEqual(['b', 'a-new'])
+  })
 })
 
 describe('workspaceCwdForNewSession', () => {

apps/desktop/src/store/session.ts

@@ -125,10 +125,18 @@ export function mergeSessionPage(
   }
 
   const incomingIds = new Set(incoming.map(session => session.id))
+  // Deduplicate by compression lineage: when auto-compression rotates the tip
+  // id (old #4 → new #5), the incoming page carries the new tip but the
+  // previous list still holds the old one.  Without lineage-level dedup both
+  // rows survive as separate sidebar entries (fixes #43483).
+  const incomingLineageKeys = new Set(
+    incoming.map(session => session._lineage_root_id ?? session.id)
+  )
 
   const survivors = previous.filter(
     session =>
       !incomingIds.has(session.id) &&
+      !incomingLineageKeys.has(session._lineage_root_id ?? session.id) &&
       (keep.has(session.id) || (session._lineage_root_id != null && keep.has(session._lineage_root_id)))
   )