Iterate design of right panel empty state (#12796)

* Add reusable empty state for the right panel

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Update tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Improve coverage

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

---------

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

test/components/structures/FilePanel-test.tsx

@@ -0,0 +1,58 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from "react";
+import { EventTimelineSet, PendingEventOrdering, Room } from "matrix-js-sdk/src/matrix";
+import { screen, render, waitFor } from "@testing-library/react";
+import { mocked } from "jest-mock";
+
+import FilePanel from "../../../src/components/structures/FilePanel";
+import ResizeNotifier from "../../../src/utils/ResizeNotifier";
+import { stubClient } from "../../test-utils";
+import { MatrixClientPeg } from "../../../src/MatrixClientPeg";
+
+jest.mock("matrix-js-sdk/src/matrix", () => ({
+    ...jest.requireActual("matrix-js-sdk/src/matrix"),
+    TimelineWindow: jest.fn().mockReturnValue({
+        load: jest.fn().mockResolvedValue(null),
+        getEvents: jest.fn().mockReturnValue([]),
+        canPaginate: jest.fn().mockReturnValue(false),
+    }),
+}));
+
+describe("FilePanel", () => {
+    beforeEach(() => {
+        stubClient();
+    });
+
+    it("renders empty state", async () => {
+        const cli = MatrixClientPeg.safeGet();
+        const room = new Room("!room:server", cli, cli.getSafeUserId(), {
+            pendingEventOrdering: PendingEventOrdering.Detached,
+        });
+        const timelineSet = new EventTimelineSet(room);
+        room.getOrCreateFilteredTimelineSet = jest.fn().mockReturnValue(timelineSet);
+        mocked(cli.getRoom).mockReturnValue(room);
+
+        const { asFragment } = render(
+            <FilePanel roomId={room.roomId} onClose={jest.fn()} resizeNotifier={new ResizeNotifier()} />,
+        );
+        await waitFor(() => {
+            expect(screen.getByText("No files visible in this room")).toBeInTheDocument();
+        });
+        expect(asFragment()).toMatchSnapshot();
+    });
+});