Trying to resize a dockwidget with a fixed width will let the splitter move ca 2px and leave the area it moves away from unpainted.

#include <QtGui>

class myDock : public QDockWidget
{
public: myDock(QWidget *parent) : QDockWidget(parent){}
QSize sizeHint() const

};

int main(int argc, char **argv)
{
QApplication app(argc, argv);
QMainWindow window;
myDock *dock1 = new myDock(&window);
window.addDockWidget(Qt::RightDockWidgetArea, dock1);

dock1->setFixedSize(300, 200);
dock1->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
dock1->setWindowFlags(dock1->windowFlags() | Qt::MSWindowsFixedSizeDialogHint);

QLabel *label = new QLabel("This is a label", dock1);
dock1->setWidget(label);
QTextEdit *edit = new QTextEdit(&window);
window.setCentralWidget(edit);
window.show();
return app.exec();
}