Room list: avoid excessive re-renders on room list store update or filter change (#32663)

* perf(room list): clear room list item vm only when changing space

Clearing all the item vms at every room list change is causing massive
re-render of all the room list items. References to the vms are already
removed when out of view (see RoomListViewMode.updateVisibleRooms) and
handled by GC.

* perf(room list): avoid to re-render filters at every room list update

RoomListView re-renders on update but the filters (children) don't need
to.

Add a memo to avoid excessive-rerenders.

* chore: add `keepIfSame` to do diff on vm objects

* perf(room list): avoid to create new filter ids and keys when the room list is updated

The filterKeys are passed in the virtuoso context so it should reduce
internal computing

The filter ids array has always the same value, there is no point to
create a new instance.

* test(room list): remove no longer relevant test

* test(room list): add new test to ensure that room list item vms are preserved

apps/web/test/unit-tests/utils/keepIfSame-test.ts

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+import { keepIfSame } from "../../../src/utils/keepIfSame";
+
+describe("keepIfSame", () => {
+    it("returns the next value if the current and next values are not deeply equal", () => {
+        const current = { a: 1 };
+        const next = { a: 2 };
+        expect(keepIfSame(current, next)).toBe(next);
+    });
+
+    it("returns the current value if the current and next values are deeply equal", () => {
+        const current = { a: 1 };
+        const next = { a: 1 };
+        expect(keepIfSame(current, next)).toBe(current);
+    });
+});

apps/web/test/viewmodels/room-list/RoomListViewViewModel-test.tsx

@@ -124,6 +124,20 @@ describe("RoomListViewViewModel", () => {
 
             expect(viewModel.getSnapshot().isLoadingRooms).toBe(false);
         });
+
+        // This test ensures that the room list item vms are preserved when the room list is changing
+        it("should keep existing view model when ListsUpdate event fires", () => {
+            viewModel = new RoomListViewViewModel({ client: matrixClient });
+
+            // Create view model for room1
+            const room1VM = viewModel.getRoomItemViewModel("!room1:server");
+            expect(room1VM).toBeDefined();
+
+            RoomListStoreV3.instance.emit(RoomListStoreV3Event.ListsUpdate);
+
+            // View model should be still valid
+            expect(room1VM.isDisposed).toBe(false);
+        });
     });
 
     describe("Space switching", () => {
@@ -295,24 +309,6 @@ describe("RoomListViewViewModel", () => {
             expect(viewModel.getSnapshot().activeFilterId).toBeUndefined();
             expect(viewModel.getSnapshot().roomIds).toEqual(["!room1:server", "!room2:server", "!room3:server"]);
         });
-
-        it("should clear view models when filter changes", () => {
-            viewModel = new RoomListViewViewModel({ client: matrixClient });
-
-            // Get view models
-            const vm1 = viewModel.getRoomItemViewModel("!room1:server");
-            const disposeSpy = jest.spyOn(vm1, "dispose");
-
-            jest.spyOn(RoomListStoreV3.instance, "getSortedRoomsInActiveSpace").mockReturnValue({
-                spaceId: "home",
-                rooms: [room2],
-                filterKeys: [FilterKey.UnreadFilter],
-            });
-
-            viewModel.onToggleFilter("unread");
-
-            expect(disposeSpy).toHaveBeenCalled();
-        });
     });
 
     describe("Room item view models", () => {