Cache calls to removeHiddenChars() to fix performance bottleneck in Safari (#3066)

* Cache calls to removeHiddenChars() as very slow on Safari

Fixes #3065

* Test

* Split testing for removeHiddenChars

src/utils/displayname-integration.test.ts

@@ -0,0 +1,40 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { afterEach, beforeAll, describe, expect, test, vi } from "vitest";
+
+import { shouldDisambiguate } from "./displayname";
+import { alice } from "./test-fixtures";
+import { mockMatrixRoom } from "./test";
+
+// Ideally these tests would be in ./displayname.test.ts but I can't figure out how to
+// just spy on the removeHiddenChars() function without impacting the other tests.
+// So, these tests are in this separate test file.
+vi.mock("matrix-js-sdk/src/utils");
+
+describe("shouldDisambiguate", () => {
+  // eslint-disable-next-line @typescript-eslint/consistent-type-imports
+  let jsUtils: typeof import("matrix-js-sdk/src/utils");
+
+  beforeAll(async () => {
+    jsUtils = await import("matrix-js-sdk/src/utils");
+    vi.spyOn(jsUtils, "removeHiddenChars").mockImplementation((str) => str);
+  });
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  test("should only call removeHiddenChars once for a single displayname", () => {
+    const room = mockMatrixRoom({});
+    shouldDisambiguate(alice, [], room);
+    expect(jsUtils.removeHiddenChars).toHaveBeenCalledTimes(1);
+    for (let i = 0; i < 10; i++) {
+      shouldDisambiguate(alice, [], room);
+    }
+    expect(jsUtils.removeHiddenChars).toHaveBeenCalledTimes(1);
+  });
+});