src/vs/base/browser/dom.ts/getContentWidth
export function getContentWidth(element: HTMLElement): number {
	let border = SizeUtils.getBorderLeftWidth(element) + SizeUtils.getBorderRightWidth(element);
	let padding = SizeUtils.getPaddingLeft(element) + SizeUtils.getPaddingRight(element);
	return element.offsetWidth - border - padding;
}