How disable scrollbar when `embedMode: "FULL_WINDOW"`?

It is impossible to hide the scrollbar, but you can get the scrollbar size and count it:

function getScrollbarWidth () {

// Creating invisible container
const outer = document.createElement('div');
outer.style.visibility = 'hidden';
outer.style.overflow = 'scroll'; // forcing scrollbar to appear
outer.style.msOverflowStyle = 'scrollbar'; // needed for WinJS apps
document.body.appendChild(outer);

// Creating inner element and placing it in the container
const inner = document.createElement('div');
outer.appendChild(inner);

// Calculating difference between container's full width and the child width
const scrollbarWidth = (outer.offsetWidth - inner.offsetWidth);

// Removing temporary elements from the DOM
outer.parentNode.removeChild(outer);

return scrollbarWidth;

}

source: https://stackoverflow.com/a/13382873/2908501