From c66d873ff59720e82b24bf0eb309fabd80dedc0e Mon Sep 17 00:00:00 2001
From: OrcaWin <alpha-eng@stably.ai>
Date: Wed, 28 Oct 2026 14:29:41 -0700
Subject: [PATCH] Reuse Explorer watcher child indexes across unchanged
 payloads (#35319)

* perf(explorer): reuse watcher child indexes by listing identity

* test(explorer): trim child index reuse tests to reuse and invalidation

---------

Co-authored-by: OrcaWin <293788423+OrcaWin@users.noreply.github.com>
---
 .../file-explorer-watch-child-index.test.ts   | 73 +++++++++++++++++++
 .../file-explorer-watch-reconcile.ts          | 23 +++---
 2 files changed, 85 insertions(+), 11 deletions(-)
 create mode 100644 src/renderer/src/components/right-sidebar/file-explorer-watch-child-index.test.ts

diff --git a/src/renderer/src/components/right-sidebar/file-explorer-watch-child-index.test.ts b/src/renderer/src/components/right-sidebar/file-explorer-watch-child-index.test.ts
new file mode 100644
index 00000000000..ef6eea7ab72
--- /dev/null
+++ b/src/renderer/src/components/right-sidebar/file-explorer-watch-child-index.test.ts
@@ -0,0 +1,73 @@
+import { describe, expect, it, vi } from 'vitest'
+import type { DirCache, TreeNode } from './file-explorer-types'
+import { processFileExplorerFsPayload } from './file-explorer-watch-reconcile'
+
+function reconcile(
+  cache: Record<string, DirCache>,
+  absolutePath: string
+): { dir: ReturnType<typeof vi.fn>; tree: ReturnType<typeof vi.fn> } {
+  const dir = vi.fn()
+  const tree = vi.fn()
+  processFileExplorerFsPayload({
+    payload: {
+      worktreePath: '/repo',
+      events: [{ kind: 'update', absolutePath, isDirectory: false }]
+    },
+    currentWorktreePath: '/repo',
+    worktreeId: 'wt-1',
+    cache,
+    expanded: new Set(),
+    setDirCache: vi.fn(),
+    setSelectedPath: vi.fn(),
+    refreshDir: dir,
+    refreshTree: tree
+  })
+  return { dir, tree }
+}
+
+function node(path: string): TreeNode {
+  return {
+    path,
+    name: path,
+    relativePath: path.slice('/repo/'.length),
+    isDirectory: false,
+    isSymlink: false,
+    depth: 0,
+    operationOwner: { kind: 'local' }
+  }
+}
+
+describe('watcher sibling index reuse', () => {
+  it('indexes unchanged siblings once across update payloads', () => {
+    let reads = 0
+    const children = Array.from({ length: 1_000 }, (_, index) => {
+      const path = `/repo/file-${index}.ts`
+      const child = node(path)
+      Object.defineProperty(child, 'path', {
+        get: () => {
+          reads++
+          return path
+        }
+      })
+      return child
+    })
+    const cache = { '/repo': { children } }
+    for (let payload = 0; payload < 10; payload++) {
+      const refresh = reconcile(cache, '/repo/file-0.ts')
+      expect(refresh.dir).not.toHaveBeenCalled()
+      expect(refresh.tree).not.toHaveBeenCalled()
+    }
+    expect(reads).toBe(1_000)
+  })
+
+  it('reindexes replaced listings while reusing an unchanged array through cache copies', () => {
+    const children = [node('/repo/old.ts')]
+    const before = { '/repo': { children } }
+    expect(reconcile(before, '/repo/old.ts').dir).not.toHaveBeenCalled()
+    expect(reconcile({ ...before }, '/repo/old.ts').dir).not.toHaveBeenCalled()
+
+    const after = { '/repo': { children: [node('/repo/new.ts')] } }
+    expect(reconcile(after, '/repo/new.ts').dir).not.toHaveBeenCalled()
+    expect(reconcile(after, '/repo/old.ts').dir).toHaveBeenCalledWith('/repo')
+  })
+})
diff --git a/src/renderer/src/components/right-sidebar/file-explorer-watch-reconcile.ts b/src/renderer/src/components/right-sidebar/file-explorer-watch-reconcile.ts
index b82b38bf76d..2da75cc7d01 100644
--- a/src/renderer/src/components/right-sidebar/file-explorer-watch-reconcile.ts
+++ b/src/renderer/src/components/right-sidebar/file-explorer-watch-reconcile.ts
@@ -31,21 +31,23 @@ export type ProcessFileExplorerFsPayloadArgs = {
   refreshTree: () => void
 }
 
+const cachedChildPathIndexes = new WeakMap<readonly TreeNode[], ReadonlyMap<string, TreeNode>>()
+
 function cachedDirectoryChild(
   cache: Record<string, DirCache>,
   cachedDirPath: string,
-  childPath: string,
-  childPathIndexes: Map<string, Map<string, TreeNode>>
+  childPath: string
 ): TreeNode | undefined {
-  let childPaths = childPathIndexes.get(cachedDirPath)
+  const children = cache[cachedDirPath]?.children
+  if (!children) {
+    return undefined
+  }
+  let childPaths = cachedChildPathIndexes.get(children)
   if (!childPaths) {
     childPaths = new Map(
-      cache[cachedDirPath]?.children.map((child) => [
-        normalizeRuntimePathForComparison(child.path),
-        child
-      ]) ?? []
+      children.map((child) => [normalizeRuntimePathForComparison(child.path), child])
     )
-    childPathIndexes.set(cachedDirPath, childPaths)
+    cachedChildPathIndexes.set(children, childPaths)
   }
   return childPaths.get(normalizeRuntimePathForComparison(childPath))
 }
@@ -83,7 +85,6 @@ export function processFileExplorerFsPayload(args: ProcessFileExplorerFsPayloadA
   }
 
   const dirsToRefresh = new Set<string>()
-  const childPathIndexes = new Map<string, Map<string, TreeNode>>()
   let hasLinkedCache: boolean | undefined
   let cachedDirKeys: string[] | undefined
   let cachedDirPathIndex: ReadonlyMap<string, string> | undefined
@@ -114,7 +115,7 @@ export function processFileExplorerFsPayload(args: ProcessFileExplorerFsPayloadA
     const cachedParent = resolveCachedDirPath(cache, parent, currentWorktreePath, cachePathIndex)
     const updatedChild =
       evt.kind === 'update' && cachedParent
-        ? cachedDirectoryChild(cache, cachedParent, normalizedPath, childPathIndexes)
+        ? cachedDirectoryChild(cache, cachedParent, normalizedPath)
         : undefined
     const knownFileUpdate =
       evt.kind === 'update' &&
@@ -227,7 +228,7 @@ export function processFileExplorerFsPayload(args: ProcessFileExplorerFsPayloadA
       if (
         cachedParent &&
         !dirsToRefresh.has(cachedParent) &&
-        !cachedDirectoryChild(cache, cachedParent, normalizedPath, childPathIndexes)
+        !cachedDirectoryChild(cache, cachedParent, normalizedPath)
       ) {
         dirsToRefresh.add(cachedParent)
       }