From a3919faed4a125f333fa4db6b23e0cdbd9dfcccd Mon Sep 17 00:00:00 2001
From: Mohamed Boudra <boudra.moha@gmail.com>
Date: Fri, 16 Oct 2026 15:18:08 +0200
Subject: [PATCH] perf(app): preserve unchanged sidebar workspace projections

Reopening a retained sidebar rebuilt its entry collection and invalidated every project row despite unchanged workspace data. Preserve collection identity as well as row identity, while keeping membership and ordering changes observable.
---
 .../sidebar-workspaces-view-model.test.ts     | 51 +++++++++++++++++++
 .../hooks/sidebar-workspaces-view-model.ts    | 21 +++++---
 2 files changed, 66 insertions(+), 6 deletions(-)

diff --git a/packages/app/src/hooks/sidebar-workspaces-view-model.test.ts b/packages/app/src/hooks/sidebar-workspaces-view-model.test.ts
index 637d3a035b..1bc4cf2135 100644
--- a/packages/app/src/hooks/sidebar-workspaces-view-model.test.ts
+++ b/packages/app/src/hooks/sidebar-workspaces-view-model.test.ts
@@ -403,6 +403,57 @@ describe("shared sidebar workspace model", () => {
     expect(model.projectNamesByViewKey).toEqual(new Map([["getpaseo/paseo", "getpaseo/paseo"]]));
   });
 
+  it("preserves the collection when a retained sidebar reconnects to unchanged entries", () => {
+    const model = buildSidebarWorkspacePlacementModel({
+      projects: [project({ projectKey: "project", workspaceKeys: ["srv:one", "srv:two"] })],
+    });
+    const one = workspace({
+      id: "one",
+      name: "one",
+      projectId: "project",
+      projectDisplayName: "project",
+    });
+    const two = workspace({
+      id: "two",
+      name: "two",
+      projectId: "project",
+      projectDisplayName: "project",
+    });
+    const sessions = [
+      {
+        serverId: "srv",
+        workspaceAgentActivity: new Map(),
+        workspaces: new Map([
+          ["one", one],
+          ["two", two],
+        ]),
+      },
+    ];
+    const previousEntries = buildSidebarWorkspaceEntries({
+      placements: model.workspaces,
+      sessions,
+    });
+    expect(
+      buildSidebarWorkspaceEntries({
+        placements: model.workspaces,
+        sessions,
+        previousEntries,
+      }),
+    ).toBe(previousEntries);
+    const reordered = buildSidebarWorkspaceEntries({
+      placements: model.workspaces.toReversed(),
+      sessions,
+      previousEntries,
+    });
+    expect([...reordered.keys()]).toEqual(["srv:two", "srv:one"]);
+    const removed = buildSidebarWorkspaceEntries({
+      placements: model.workspaces.slice(0, 1),
+      sessions,
+      previousEntries,
+    });
+    expect([...removed.keys()]).toEqual(["srv:one"]);
+  });
+
   it("preserves unchanged row identities when another workspace updates", () => {
     const model = buildSidebarWorkspacePlacementModel({
       projects: [project({ projectKey: "project", workspaceKeys: ["srv:one", "srv:two"] })],
diff --git a/packages/app/src/hooks/sidebar-workspaces-view-model.ts b/packages/app/src/hooks/sidebar-workspaces-view-model.ts
index 24ecc34461..e004c3fb89 100644
--- a/packages/app/src/hooks/sidebar-workspaces-view-model.ts
+++ b/packages/app/src/hooks/sidebar-workspaces-view-model.ts
@@ -369,13 +369,15 @@ export function buildSidebarWorkspaceEntries(input: {
   sessions: SidebarWorkspaceSession[];
   pendingCreateAttempts?: Record<string, PendingCreateAttempt>;
   previousEntries?: ReadonlyMap<string, SidebarWorkspaceEntry>;
-}): Map<string, SidebarWorkspaceEntry> {
+}): ReadonlyMap<string, SidebarWorkspaceEntry> {
   if (input.placements.length === 0 || input.sessions.length === 0) {
     return new Map();
   }
 
   const sessionByServerId = new Map(input.sessions.map((session) => [session.serverId, session]));
   const entries = new Map<string, SidebarWorkspaceEntry>();
+  const previousKeys = input.previousEntries?.keys();
+  let unchanged = input.previousEntries !== undefined;
 
   for (const placement of input.placements) {
     const session = sessionByServerId.get(placement.serverId);
@@ -395,15 +397,22 @@ export function buildSidebarWorkspaceEntries(input: {
       workspaceAgentActivity: session.workspaceAgentActivity,
     });
     const previousEntry = input.previousEntries?.get(placement.workspaceKey);
-    entries.set(
-      placement.workspaceKey,
+    const nextEntry =
       previousEntry && areSidebarWorkspaceEntriesEqual(previousEntry, entry)
         ? previousEntry
-        : entry,
-    );
+        : entry;
+    entries.set(placement.workspaceKey, nextEntry);
+    unchanged =
+      previousKeys?.next().value === placement.workspaceKey &&
+      previousEntry === nextEntry &&
+      unchanged;
   }
 
-  return entries;
+  // Resuming a retained sidebar must not invalidate its project/filter projections
+  // when the directory has not changed while it was hidden.
+  return unchanged && input.previousEntries?.size === entries.size
+    ? input.previousEntries
+    : entries;
 }
 
 function areSidebarWorkspaceEntriesEqual(