how to show button clicked image when I use QScroller::grabGesture

[mkdh]

I add a 'scrollarea' in my dialog .
I let the lay out being Horizentally in 'scrollarea'.
and I add some qpushbutton in 'scroll area'.

And my constructor:
Dialog::Dialog(QWidget *parent) :
QDialog(parent),
ui(new Ui::Dialog)
{
ui->setupUi(this);
QScroller::grabGesture(ui->scrollArea,QScroller::LeftMouseButtonGesture);
}

my question is:
it don't show the clicked button image when I click the button.
Why?and how to fixed it?