longresize.html

<!DOCTYPE html>

<html>
    <head>
        <meta name="viewport" content="width=device-width">

        <style>
            html, body {
                margin: 0px;
                width: 100%;
                height: 100%;
            }
            
            #resizeinfo {
                width: 100px;
                height: 200px;
            }
            
        </style>
        <script>        
            function sleep(ms) {
              return new Promise(resolve => setTimeout(resolve, ms));
            }

            async function blockFor(delayTimeMs) {
              await sleep(delayTimeMs);
            }
            function didResize() {
              console.log("Resize Handler...");
              blockFor(3000);     
              console.log("Done sleep");
              var resizebox = document.getElementById("resizeinfo");
              resizebox.innerHTML += "RESIZE!<br>"
              
              
            }
            window.addEventListener('load', function() {
                window.addEventListener('resize', didResize);
             
            });
        </script>
    </head>

    <body>
        <div id="resizeinfo"></div>
    </body>
</html>