QFontMetricsF::boundingRect handles a null QRectF differently when passed in as it does not constrain to the size of it

QFontMetricsF::boundingRect handles a null QRectF differently when passed in as it does not constrain to the size of it, this would appear to be because it is a null rect and therefore it handles it in a different way. So this should be documented to be the case.