QFontMetrics::boundingRect(QString) returns incorrect bounding box on Windows XP and Mac OSX

The bounding boxes of QStrings as returned by QFontMetrics::boundingRect(QString) tend to be a couple of pixels wider then the enclosed text. This is apparent under Windows XP and on OSX.

Note: In Qt 4.3 there is a call giving the exact bounding Box of a string called tightBoundingBox(). The main problem here is that the method is very slow on Windows.