Fix thread summary layout for narrow right panel timeline (#7838)

src/components/views/elements/Measured.tsx

@@ -0,0 +1,71 @@
+/*
+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 UIStore, { UI_EVENTS } from "../../../stores/UIStore";
+
+interface IProps {
+    sensor: Element;
+    breakpoint: number;
+    onMeasurement(narrow: boolean): void;
+}
+
+export default class Measured extends React.PureComponent<IProps> {
+    private static instanceCount = 0;
+    private readonly instanceId: number;
+
+    static defaultProps = {
+        breakpoint: 500,
+    };
+
+    constructor(props) {
+        super(props);
+
+        this.instanceId = Measured.instanceCount++;
+    }
+
+    componentDidMount() {
+        UIStore.instance.on(`Measured${this.instanceId}`, this.onResize);
+    }
+
+    componentDidUpdate(prevProps: Readonly<IProps>) {
+        const previous = prevProps.sensor;
+        const current = this.props.sensor;
+        if (previous === current) return;
+        if (previous) {
+            UIStore.instance.stopTrackingElementDimensions(`Measured${this.instanceId}`);
+        }
+        if (current) {
+            UIStore.instance.trackElementDimensions(`Measured${this.instanceId}`,
+                this.props.sensor);
+        }
+    }
+
+    componentWillUnmount() {
+        UIStore.instance.off(`Measured${this.instanceId}`, this.onResize);
+        UIStore.instance.stopTrackingElementDimensions(`Measured${this.instanceId}`);
+    }
+
+    private onResize = (type: UI_EVENTS, entry: ResizeObserverEntry) => {
+        if (type !== UI_EVENTS.Resize) return;
+        this.props.onMeasurement(entry.contentRect.width <= this.props.breakpoint);
+    };
+
+    render() {
+        return null;
+    }
+}