Try harder to keep context menus inside the window

src/components/structures/ContextMenu.tsx

@@ -31,6 +31,7 @@ import { checkInputableElement, RovingTabIndexProvider } from "../../accessibili
 // of doing reusable widgets like dialog boxes & menus where we go and
 // pass in a custom control as the actual body.
 
+const WINDOW_PADDING = 10;
 const ContextualMenuContainerId = "mx_ContextualMenu_Container";
 
 function getOrCreateContainer(): HTMLDivElement {
@@ -247,21 +248,49 @@ export default class ContextMenu extends React.PureComponent<IProps, IState> {
 
         if (chevronFace === ChevronFace.Top || chevronFace === ChevronFace.Bottom) {
             chevronOffset.left = props.chevronOffset;
-        } else if (position.top !== undefined) {
-            const target = position.top;
-
-            // By default, no adjustment is made
-            let adjusted = target;
+        } else {
+            chevronOffset.top = props.chevronOffset;
+        }
 
-            // If we know the dimensions of the context menu, adjust its position
-            // such that it does not leave the (padded) window.
-            if (contextMenuRect) {
-                const padding = 10;
-                adjusted = Math.min(position.top, document.body.clientHeight - contextMenuRect.height - padding);
+        // If we know the dimensions of the context menu, adjust its position to
+        // keep it within the bounds of the (padded) window
+        const { windowWidth, windowHeight } = UIStore.instance;
+        if (contextMenuRect) {
+            if (position.top !== undefined) {
+                position.top = Math.min(
+                    position.top,
+                    windowHeight - contextMenuRect.height - WINDOW_PADDING,
+                );
+                // Adjust the chevron if necessary
+                if (chevronOffset.top !== undefined) {
+                    chevronOffset.top = props.chevronOffset + props.top - position.top;
+                }
+            } else if (position.bottom !== undefined) {
+                position.bottom = Math.min(
+                    position.bottom,
+                    windowHeight - contextMenuRect.height - WINDOW_PADDING,
+                );
+                if (chevronOffset.top !== undefined) {
+                    chevronOffset.top = props.chevronOffset + props.bottom - position.bottom;
+                }
+            }
+            if (position.left !== undefined) {
+                position.left = Math.min(
+                    position.left,
+                    windowWidth - contextMenuRect.width - WINDOW_PADDING,
+                );
+                if (chevronOffset.left !== undefined) {
+                    chevronOffset.left = props.chevronOffset + props.left - position.left;
+                }
+            } else if (position.right !== undefined) {
+                position.right = Math.min(
+                    position.right,
+                    windowWidth - contextMenuRect.width - WINDOW_PADDING,
+                );
+                if (chevronOffset.left !== undefined) {
+                    chevronOffset.left = props.chevronOffset + props.right - position.right;
+                }
             }
-
-            position.top = adjusted;
-            chevronOffset.top = Math.max(props.chevronOffset, props.chevronOffset + target - adjusted);
         }
 
         let chevron;

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);
+    });
+});