Extracted from `timoxley/offset`, but directly using a
TextRectangle instead of getting another version.

@param {TextRectangle} box - result from a `getBoundingClientRect()` call
@param {Document} doc - Document instance to use
@return {Object} an object with `top` and `left` Number properties
@api private
function _offset( box, doc ) {
	const body = doc.body || doc.getElementsByTagName( 'body' )[ 0 ];
	const docEl = doc.documentElement || body.parentNode;
	const clientTop = docEl.clientTop || body.clientTop || 0;
	const clientLeft = docEl.clientLeft || body.clientLeft || 0;
	const scrollTop = window.pageYOffset || docEl.scrollTop;
	const scrollLeft = window.pageXOffset || docEl.scrollLeft;

	return {
		top: box.top + scrollTop - clientTop,
		left: box.left + scrollLeft - clientLeft,
	};
}