From 5dbd23364bc8ccce9568d74f13e4a3d13534c083 Mon Sep 17 00:00:00 2001
From: OrcaWin <alpha-eng@stably.ai>
Date: Thu, 22 Oct 2026 13:20:02 -0700
Subject: [PATCH] perf(layout): index records once while patching session lists
 (#31935)

Co-authored-by: OrcaWin <293788423+OrcaWin@users.noreply.github.com>
---
 .../workspace-layout-record-patch.ts          | 26 +++++++++++++++----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/src/shared/workspace-layout/workspace-layout-record-patch.ts b/src/shared/workspace-layout/workspace-layout-record-patch.ts
index f16cfc8d417..72811e643d7 100644
--- a/src/shared/workspace-layout/workspace-layout-record-patch.ts
+++ b/src/shared/workspace-layout/workspace-layout-record-patch.ts
@@ -103,8 +103,20 @@ function patchLists(
   return patched
 }
 
+function indexRecordsById(list: Json[], idOf: (record: Json) => unknown): Map<unknown, Json> {
+  const records = new Map<unknown, Json>()
+  for (const record of list) {
+    const id = idOf(record)
+    if (!records.has(id)) {
+      records.set(id, record)
+    }
+  }
+  return records
+}
+
 function patchList(stored: Json[], before: Json[], after: Json[], idOf: (record: Json) => unknown) {
-  const find = (list: Json[], id: unknown) => list.find((record) => idOf(record) === id)
+  const beforeById = indexRecordsById(before, idOf)
+  const afterById = indexRecordsById(after, idOf)
   const result = [...stored]
   const ids = new Set([...before.map(idOf), ...after.map(idOf)])
   const kept = (list: Json[], other: Json[]) => {
@@ -114,8 +126,8 @@ function patchList(stored: Json[], before: Json[], after: Json[], idOf: (record:
   // Records kept their relative order: sort orders changed only to close a removal's gap.
   const renumberedOnly = sameJson(kept(before, after), kept(after, before))
   for (const id of ids) {
-    const old = find(before, id)
-    const next = find(after, id)
+    const old = beforeById.get(id)
+    const next = afterById.get(id)
     if (sameJson(old, next)) {
       continue
     }
@@ -147,10 +159,14 @@ function patchList(stored: Json[], before: Json[], after: Json[], idOf: (record:
   }
   // The command reordered its own records: they take `after`'s order in the slots they hold.
   const placed = new Set(after.map(idOf))
+  const resultById = indexRecordsById(result, idOf)
   const queue = after
-    .map((record) => result.find((candidate) => idOf(candidate) === idOf(record)))
+    .map((record) => resultById.get(idOf(record)))
     .filter((record): record is Json => record !== undefined)
-  return result.map((record) => (placed.has(idOf(record)) ? (queue.shift() ?? record) : record))
+  let nextIndex = 0
+  return result.map((record) =>
+    placed.has(idOf(record)) ? (queue[nextIndex++] ?? record) : record
+  )
 }
 
 /** After the nearest record that precedes it in `after` and is already in the list. */