Create flexbox layout helper (#11411)

src/components/utils/Box.tsx

@@ -0,0 +1,91 @@
+/*
+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 classNames from "classnames";
+import React, { useEffect, useRef } from "react";
+
+type FlexProps = {
+    /**
+     * The type of the HTML element
+     * @default div
+     */
+    as?: string;
+    /**
+     * The CSS class name.
+     */
+    className?: string;
+    /**
+     * the on click event callback
+     */
+    onClick?: (e: React.MouseEvent) => void;
+    /**
+     * The flex space to use
+     * @default null
+     */
+    flex?: string | null;
+    /**
+     * The flex shrink factor
+     * @default null
+     */
+    shrink?: string | null;
+    /**
+     * The flex grow factor
+     * @default null
+     */
+    grow?: string | null;
+};
+
+/**
+ * Set or remove a CSS property
+ * @param ref the reference
+ * @param name the CSS property name
+ * @param value the CSS property value
+ */
+function addOrRemoveProperty(
+    ref: React.MutableRefObject<HTMLElement | undefined>,
+    name: string,
+    value?: string | null,
+): void {
+    const style = ref.current!.style;
+    if (value) {
+        style.setProperty(name, value);
+    } else {
+        style.removeProperty(name);
+    }
+}
+
+/**
+ * A flex child helper
+ */
+export function Box({
+    as = "div",
+    flex = null,
+    shrink = null,
+    grow = null,
+    className,
+    children,
+    ...props
+}: React.PropsWithChildren<FlexProps>): JSX.Element {
+    const ref = useRef<HTMLElement>();
+
+    useEffect(() => {
+        addOrRemoveProperty(ref, `--mx-box-flex`, flex);
+        addOrRemoveProperty(ref, `--mx-box-shrink`, shrink);
+        addOrRemoveProperty(ref, `--mx-box-grow`, grow);
+    }, [flex, grow, shrink]);
+
+    return React.createElement(as, { ...props, className: classNames("mx_Box", className), ref }, children);
+}