Allow maintaining a different right panel width for thread panels (#11064)

* Move Room context threadId our of RoomView state

* Allow maintaining a different right panel width for thread panels

* Fix types

* Fix types

* Add tests

* Increase coverage

* Increase coverage

* Add comments

* Update src/components/structures/MainSplit.tsx

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

test/components/structures/MainSplit-test.tsx

@@ -0,0 +1,63 @@
+/*
+Copyright 2023 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 { render } from "@testing-library/react";
+
+import MainSplit from "../../../src/components/structures/MainSplit";
+import ResizeNotifier from "../../../src/utils/ResizeNotifier";
+
+describe("<MainSplit/>", () => {
+    const resizeNotifier = new ResizeNotifier();
+    const children = (
+        <div>
+            Child<span>Foo</span>Bar
+        </div>
+    );
+    const panel = <div>Right panel</div>;
+
+    it("renders", () => {
+        const { asFragment, container } = render(
+            <MainSplit resizeNotifier={resizeNotifier} children={children} panel={panel} />,
+        );
+        expect(asFragment()).toMatchSnapshot();
+        // Assert it matches the default width of 350
+        expect(container.querySelector<HTMLElement>(".mx_RightPanel_ResizeWrapper")!.style.width).toBe("350px");
+    });
+
+    it("respects defaultSize prop", () => {
+        const { asFragment, container } = render(
+            <MainSplit resizeNotifier={resizeNotifier} children={children} panel={panel} defaultSize={500} />,
+        );
+        expect(asFragment()).toMatchSnapshot();
+        // Assert it matches the default width of 350
+        expect(container.querySelector<HTMLElement>(".mx_RightPanel_ResizeWrapper")!.style.width).toBe("500px");
+    });
+
+    it("prefers size stashed in LocalStorage to the defaultSize prop", () => {
+        localStorage.setItem("mx_rhs_size_thread", "333");
+        const { container } = render(
+            <MainSplit
+                resizeNotifier={resizeNotifier}
+                children={children}
+                panel={panel}
+                sizeKey="thread"
+                defaultSize={400}
+            />,
+        );
+        expect(container.querySelector<HTMLElement>(".mx_RightPanel_ResizeWrapper")!.style.width).toBe("333px");
+    });
+});