getRemainingHeight = ( element, height = 0 ) => {

	if( element ) {
		let newHeight, oldHeight = element.style.height;

		// Change the .stretch element height to 0 in order find the height of all
		// the other elements
		element.style.height = '0px';

		// In Overview mode, the parent (.slide) height is set of 700px.
		// Restore it temporarily to its natural height.
		element.parentNode.style.height = 'auto';

		newHeight = height - element.parentNode.offsetHeight;

		// Restore the old height, just in case
		element.style.height = oldHeight + 'px';

		// Clear the parent (.slide) height. .removeProperty works in IE9+
		element.parentNode.style.removeProperty('height');

		return newHeight;
	}

	return height;

}
Returns the remaining height within the parent of the
target element.

remaining height = [ configured parent height ] - [ current parent height ]

@param {HTMLElement} element
@param {number} [height]