Touchscreen: QDialog with Qt::FramelessWindowHint faulty

Error404

OS: FriendlyDesktop with X Window (based on Ubuntu 18.04 64bit)
Device: NanoPi NEO4
Qt Version: 5.10.0

Hi!

I just want to use my QApplication with a touchscreen. My MainWindow is on fullscreen and works perfectly with around 10 different buttons.
If I open a new QDialog with the Window Flag "Qt::FramelessWindowHint" the buttons on the QDialog are not working in at least 80% of all cases (not pressable, no highlighting,...). I enabled the cursor to see if the touchscreen works => works fine. I connected with VNC to press the button with a mouse => works fine.

If I don't use "Qt::FramelessWindowHint" everything is working, too. The problem is, that the buttons on the titlebar have to be disabled! f.e.: If I press minimize, the QDialog goes into the background and I can't do anything on the MainWindow anymore, which is kind of a bottleneck, because then I can't even shutdown the device with just the touchscreen ...
I found no working solution to disable the buttons in the titlebar for linux. Either the entire title bar is deactivated or nothing at all...

I think my application isn't out of ordinary so there has to be a solution.

Thank you!

Error404

Update: It looks like the buttons on the QDialog are just moved sometimes...annoying

This happend just with "Qt::FramelessWindowHint"!

Error404

Update: Now I just use a QWidget instead of a QDialog and everything is working fine...

dialogwidget.h

signals:
    void WidgetClosed(bool);

dialogwidget.cpp:

void deletewindowusurewidget::on_btnabort_clicked()
{
    emit WidgetClosed(false);
}

void deletewindowusurewidget::on_btnok_clicked()
{
    emit WidgetClosed(true);
}

mainwindow.h

private:
    dialogwidget *usure = new dialogwidget(this);

mainwindow.cpp

    //constructor:
    connect(usure, SIGNAL(WidgetClosed(bool)), this, SLOT(dialogwidgetclosed(bool)));

    //start dialog
    usure->setWindowModality(Qt::ApplicationModal);
    usure->setWindowFlags(Qt::Tool | Qt::Dialog | Qt::FramelessWindowHint);
    usure->move(this->rect().center() - usure->rect().center());
    usure->show();

    //finished signal
    void MainWindow::deletewindowusureclosed(bool deleteall)
    {
         usure->close();
         if(deleteall)
         {
             //btnok pressed
         }
         else
         {
             //btnabort pressed
         }

    }