I've tried to follow your advice but it doesn't work
Qt Code: 
  1. class MyStyle : public ProxyStyle
  2.  {
  3.  public:
  4.      MyStyle(const QString &baseStyle);
  5.  
  6.      int pixelMetric(PixelMetric metric, const QStyleOption* option = 0,
  7. 					 const QWidget* widget = 0) const;
  8.  
  9. 	QIcon standardIcon(StandardPixmap standardIcon, const QStyleOption* option = 0,
  10. 					   const QWidget* widget = 0) const;
  11.  };
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Qt Code: 
  1. int MyStyle::pixelMetric(PixelMetric metric, const QStyleOption* option,
  2. 					 const QWidget* widget) const
  3. {
  4. 	if (metric == PM_DockWidgetHandleExtent || metric == PM_DockWidgetSeparatorExtent)
  5. 	{
  6. 		qDebug()<<"PM_DockWidgetHandleExtent";
  7. 		return 110;
  8. 	}
  9. 	else
  10. 		return ProxyStyle::pixelMetric(metric, option, widget);
  11. }
  12.  
  13. QIcon MyStyle::standardIcon(StandardPixmap standardIcon, const QStyleOption* option,
  14. 					   const QWidget* widget) const
  15. {
  16. 	if(standardIcon == SP_TitleBarNormalButton)
  17. 		return QIcon(QPixmap(":/resources/backfordropdown.jpg"));
  18. 	else
  19. 		return ProxyStyle::standardIcon(standardIcon, option, widget);
  20. }
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