diff --git a/src/renderer/src/components/sidebar/worktree-lineage-expansion.performance.test.ts b/src/renderer/src/components/sidebar/worktree-lineage-expansion.performance.test.ts
new file mode 100644
index 00000000000..c6f6fdf11a1
--- /dev/null
+++ b/src/renderer/src/components/sidebar/worktree-lineage-expansion.performance.test.ts
@@ -0,0 +1,88 @@
+import { describe, expect, it, vi } from 'vitest'
+import {
+  expandDraggedWorktreeIdsForVisibleLineage,
+  type WorktreeDragLineageRow
+} from './worktree-manual-order'
+
+// Preserve the original traversal as an independent ordering oracle.
+function referenceExpansion(
+  rows: readonly WorktreeDragLineageRow[],
+  draggedIds: readonly string[]
+) {
+  const dragged = new Set(draggedIds)
+  const expanded = new Set(draggedIds)
+  const rowIds = new Set(rows.map((row) => row.worktreeId))
+  for (let index = 0; index < rows.length; index++) {
+    const row = rows[index]
+    if (!dragged.has(row.worktreeId)) {
+      continue
+    }
+    for (let cursor = index + 1; cursor < rows.length; cursor++) {
+      const child = rows[cursor]
+      if (child.depth <= row.depth) {
+        break
+      }
+      expanded.add(child.worktreeId)
+    }
+  }
+  const result = rows.filter((row) => expanded.has(row.worktreeId)).map((row) => row.worktreeId)
+  for (const id of draggedIds) {
+    if (!rowIds.has(id) && !result.includes(id)) {
+      result.push(id)
+    }
+  }
+  return result
+}
+
+describe('visible lineage expansion scaling', () => {
+  it('reads each depth once even when every ancestor is selected', () => {
+    let depthReads = 0
+    const rows = Array.from({ length: 2_000 }, (_, index) => ({
+      worktreeId: `wt-${index}`,
+      get depth() {
+        depthReads++
+        return index
+      }
+    }))
+    const selected = rows.map((row) => row.worktreeId)
+    expect(referenceExpansion(rows, selected)).toEqual(selected)
+    expect(depthReads).toBe(rows.length * (rows.length - 1))
+    depthReads = 0
+    expect(expandDraggedWorktreeIdsForVisibleLineage(rows, selected)).toEqual(selected)
+    expect(depthReads).toBe(rows.length)
+  })
+
+  it('deduplicates many absent selected rows without searching the growing result', () => {
+    const selected = Array.from({ length: 5_000 }, (_, index) => `missing-${index}`)
+    const includes = vi.spyOn(Array.prototype, 'includes')
+    let calls: number
+    let result: string[]
+    try {
+      result = expandDraggedWorktreeIdsForVisibleLineage([], [...selected, ...selected])
+      calls = includes.mock.calls.length
+    } finally {
+      includes.mockRestore()
+    }
+    expect(result).toEqual(selected)
+    expect(calls).toBe(0)
+  })
+
+  it('matches the previous algorithm across duplicate rows, missing IDs and depth boundaries', () => {
+    let seed = 12345
+    function random(limit: number): number {
+      seed = (Math.imul(seed, 1664525) + 1013904223) >>> 0
+      return seed % limit
+    }
+    const depths = [-1, 0, 1, 2, 3, 20, Infinity, -Infinity, Number.NaN]
+    for (let sample = 0; sample < 500; sample++) {
+      const rows = Array.from({ length: random(40) }, () => ({
+        worktreeId: `wt-${random(20)}`,
+        depth: depths[random(depths.length)]
+      }))
+      const selected = Array.from({ length: random(20) }, () => `wt-${random(25)}`)
+      expect(expandDraggedWorktreeIdsForVisibleLineage(rows, selected)).toEqual(
+        referenceExpansion(rows, selected)
+      )
+    }
+  })
+})
diff --git a/src/renderer/src/components/sidebar/worktree-manual-order.test.ts b/src/renderer/src/components/sidebar/worktree-manual-order.test.ts
index ca962ace9c0..6992f0f98b1 100644
--- a/src/renderer/src/components/sidebar/worktree-manual-order.test.ts
+++ b/src/renderer/src/components/sidebar/worktree-manual-order.test.ts
@@ -120,6 +120,44 @@ describe('expandDraggedWorktreeIdsForVisibleLineage', () => {
       )
     ).toEqual(['parent', 'child', 'z'])
   })
+
+  // Single-pass coverage must not end early when a descendant is also selected.
+  it('keeps ancestor coverage when a nested descendant is selected too', () => {
+    expect(
+      expandDraggedWorktreeIdsForVisibleLineage(
+        [
+          { worktreeId: 'parent', depth: 0 },
+          { worktreeId: 'child', depth: 2 },
+          { worktreeId: 'uncleish', depth: 1 },
+          { worktreeId: 'sibling', depth: 0 }
+        ],
+        ['parent', 'child']
+      )
+    ).toEqual(['parent', 'child', 'uncleish'])
+  })
+
+  it('restarts coverage for a later selected sibling after the previous run ends', () => {
+    expect(
+      expandDraggedWorktreeIdsForVisibleLineage(
+        [
+          { worktreeId: 'first', depth: 0 },
+          { worktreeId: 'first-child', depth: 1 },
+          { worktreeId: 'second', depth: 0 },
+          { worktreeId: 'second-child', depth: 1 }
+        ],
+        ['first', 'second']
+      )
+    ).toEqual(['first', 'first-child', 'second', 'second-child'])
+  })
+
+  it('emits each absent selected id once', () => {
+    expect(
+      expandDraggedWorktreeIdsForVisibleLineage(
+        [{ worktreeId: 'row', depth: 0 }],
+        ['gone', 'row', 'gone']
+      )
+    ).toEqual(['row', 'gone'])
+  })
 })
 
 describe('moveWorktreeIdsWithinGroup', () => {
diff --git a/src/renderer/src/components/sidebar/worktree-manual-order.ts b/src/renderer/src/components/sidebar/worktree-manual-order.ts
index 70978780252..887a5f17213 100644
--- a/src/renderer/src/components/sidebar/worktree-manual-order.ts
+++ b/src/renderer/src/components/sidebar/worktree-manual-order.ts
@@ -41,17 +41,18 @@ export function expandDraggedWorktreeIdsForVisibleLineage(
   const expandedSet = new Set(draggedIds)
   const rowIdSet = new Set(rows.map((row) => row.worktreeId))
 
-  for (let index = 0; index < rows.length; index++) {
-    const row = rows[index]!
-    if (!draggedSet.has(row.worktreeId)) {
-      continue
+  let ancestorDepth: number | undefined
+  for (const row of rows) {
+    const depth = row.depth
+    if (ancestorDepth !== undefined && depth <= ancestorDepth) {
+      ancestorDepth = undefined
     }
-    for (let cursor = index + 1; cursor < rows.length; cursor++) {
-      const child = rows[cursor]!
-      if (child.depth <= row.depth) {
-        break
-      }
-      expandedSet.add(child.worktreeId)
+    if (ancestorDepth !== undefined) {
+      expandedSet.add(row.worktreeId)
+    }
+    // Nested selections are already covered; NaN preserves an unterminated legacy scan.
+    if (draggedSet.has(row.worktreeId) && (ancestorDepth === undefined || Number.isNaN(depth))) {
+      ancestorDepth = depth
     }
   }
 
@@ -62,8 +63,9 @@ export function expandDraggedWorktreeIdsForVisibleLineage(
     }
   }
   for (const id of draggedIds) {
-    if (!rowIdSet.has(id) && !expandedIds.includes(id)) {
+    if (!rowIdSet.has(id)) {
       expandedIds.push(id)
+      rowIdSet.add(id)
     }
   }
   return expandedIds