Focus on the PDF viewer when it loads

To focus on the Nutrient instance when it loads, get a reference to the window object of the Nutrient Web SDK viewer frame and call focus() on it after the PSPDFKit.load promise resolves:

PSPDFKit.load(configuration).then((instance) => {
  instance.contentWindow.focus();
});

This has been tested with Nutrient Web SDK 2020.5.1.