CE_ToolButtonLabel may draw wrong icon on systems with multiple displays

On Macintosh OS X 10.10.2 (or as early as there was Retina support).

Make a QToolButton (or probably any kind of button as long as it has an icon for the label). My system has both a Retina display and a standard resolution monitor (it's a 5K iMac with external monitor). QCommonStyle::drawControl() when asked by a QToolButton to draw its label (QStyle::CE_ToolButtonLabel), uses QIcon::pixmap() to get the icon's QPixmap. The icon has both a regular icon and @2x icon, but since the call isn't the one that passes in a QWindow, it calls QApplication::devicePixelRatio() to determine the resolution. That returns the resolution of the highest-resolution monitor on the system NOT the resolution of the monitor where the button resides.

QPaintDevice::devicePixelRatio() has the correct resolution, but QIcon has no call to get a pixmap using the QPaintDevice. So one solution would be for QIcon::pixmap( ) to have an overload that takes a QPainter or QPaintDevice as input.

In the absence of the appropriate QIcon API, you can still get the right QPixmap using the overload of QIcon::pixmap() that takes a QWindow * as input. In my own code, I use this to get the QWindow *:

QWindow * QWindowHostForQWidget(const QWidget *widget)
{
	QWindow * toplevel = NULL;
	QWidget * theParent = widget->parentWidget();
	do
	{
		toplevel = theParent->windowHandle();
		theParent = theParent->parentWidget();
	} while(toplevel == NULL && theParent != NULL);
	return toplevel;
}

However it is done, though, QStyle::CE_ToolButtonLabel should draw icons that match the device's resolution when one is available.