The code below reproduces the problem mentioned in the title. The message will be cut. This problem was only observed on Windows.

Test case main.cpp to reproduce
//-------------------------------------------------------------
#include <QtGui>

int main( int argc, char * argv[] )
{
QApplication app( argc, argv );

// COMMENT THE NEXT LINE AND EVERYTHING RUNS SMOOTHLY
qApp->setStyleSheet( "QWidget

" );

QDialog dlg;
QHBoxLayout * layout = new QHBoxLayout( & dlg );
dlg.setLayout( layout );

QLabel * label = new QLabel( "Right-click me", & dlg );
label->setWhatsThis( "This WhatsThis text might miss a couple of words." );
layout->addWidget( label );

dlg.show();
return app.exec();
}
//-------------------------------------------------------------