src/vs/base/common/strings.ts/getCharContainingOffset
export function getCharContainingOffset(str: string, offset: number): [number, number] {
	if (offset > 0 && isLowSurrogate(str.charCodeAt(offset))) {
		return _getCharContainingOffset(str, offset - 1);
	}
	return _getCharContainingOffset(str, offset);
}