Code

var viewDiv = document.getElementById("view");
new ResizeObserver(viewResized).observe(viewDiv);

function viewResized()
{
    let xPixels = viewDiv.offsetWidth;
    let yPixels = viewDiv.offsetHeight;

    // convert size to diagram units
    var unitSize = diagramView.clientToDocOffset({ x: xPixels, y: yPixels });

    // don't make the scrollable area smaller
    var unitWidth = Math.max(diagram.bounds.width, unitSize.x);
    var unitHeight = Math.max(diagram.bounds.height, unitSize.y);

    diagram.bounds = new Rect(0, 0, unitWidth, unitHeight);
}