@ui5/webcomponents-react/dist/webComponents/DynamicPage/helpers.js

React Wrapper for UI5 Web Components and additional components

import { useEffect, useState } from 'react';
export const useGetHeaderHeight = dynamicPageRef => {
  const [headerHeight, setHeaderHeight] = useState(undefined);
  useEffect(() => {
    const headerObserver = new ResizeObserver(([header]) => {
      setHeaderHeight(header.contentRect.height);
    });
    if (dynamicPageRef.current) {
      // wait for the custom element to be defined (adjust the tag-name if you're using the scoping feature)
      void customElements.whenDefined('ui5-dynamic-page').then(() => {
        const {
          shadowRoot
        } = dynamicPageRef.current;

        // wait for the shadowRoot to be populated
        const shadowRootObserver = new MutationObserver(() => {
          const header = shadowRoot.querySelector('header');
          if (header) {
            shadowRootObserver.disconnect();
            headerObserver.observe(header);
          }
        });
        if (shadowRoot && shadowRoot.childElementCount) {
          headerObserver.observe(shadowRoot.querySelector('header'));
        } else {
          shadowRootObserver.observe(shadowRoot, {
            childList: true
          });
        }
      });
    }
    return () => {
      headerObserver.disconnect();
    };
  }, []);
  return headerHeight;
};