Try harder to keep context menus inside the window (#7863)

* Try harder to keep context menus inside the window

Signed-off-by: Robin Townsend <robin@robin.town>

* Use UIStore for window dimensions

Signed-off-by: Robin Townsend <robin@robin.town>

* Test ContextMenu positioning

Signed-off-by: Robin Townsend <robin@robin.town>

test/components/views/context_menus/ContextMenu-test.tsx

@@ -0,0 +1,59 @@
+/*
+Copyright 2022 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 { mount } from "enzyme";
+
+import "../../../skinned-sdk";
+import ContextMenu, { ChevronFace } from "../../../../src/components/structures/ContextMenu.tsx";
+import UIStore from "../../../../src/stores/UIStore.ts";
+
+describe("<ContextMenu />", () => {
+    // Hardcode window and menu dimensions
+    const windowSize = 300;
+    const menuSize = 200;
+    jest.spyOn(UIStore, "instance", "get").mockImplementation(() => ({
+        windowWidth: windowSize,
+        windowHeight: windowSize,
+    }));
+    window.Element.prototype.getBoundingClientRect = jest.fn().mockReturnValue({
+        width: menuSize,
+        height: menuSize,
+    });
+
+    const targetY = windowSize - menuSize + 50;
+    const targetChevronOffset = 25;
+
+    const wrapper = mount(
+        <ContextMenu
+            top={targetY}
+            left={0}
+            chevronFace={ChevronFace.Right}
+            chevronOffset={targetChevronOffset}
+        />,
+    );
+    const chevron = wrapper.find(".mx_ContextualMenu_chevron_right");
+
+    const actualY = parseInt(wrapper.getDOMNode().style.getPropertyValue("top"));
+    const actualChevronOffset = parseInt(chevron.getDOMNode().style.getPropertyValue("top"));
+
+    it("stays within the window", () => {
+        expect(actualY + menuSize).toBeLessThanOrEqual(windowSize);
+    });
+    it("positions the chevron correctly", () => {
+        expect(actualChevronOffset).toEqual(targetChevronOffset + targetY - actualY);
+    });
+});