Question

I’m implementing Qt‘s drag and drop API across Windows and X. When I pick up an object in the app running on X and drag it, it leaves a white ghost trail of itself on the window underneath, as if the window underneath is being slow to repaint where the dragged object was previously obscuring part of itself.

I believe that this is symptomatic of the same problem that Qt has just solved with resizing windows causing flicker in child widgets on X windows – i.e. the dragged object is treated as a separate native window and therefore X handles the clipping from the dragged object to the window underneath. Since X does this in a different way to Qt, we get the ghosting effect.

Has anyone experienced the same problems? One solution that comes to mind is to use the same technique as detailed in the blog article linked above and stop the dragged object being treated as a native window, presumably at the cost of drag and drop being limited to my application only (I have no problem with this). Would anyone know how to force drag and drop operations to be internal only?

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EDIT: I’m using QDrag::setPixmap to set the graphical representation of the dragged object – it is important that I retain this in favour of a standard drag cursor as this interface is being used on a touchscreen device and will hence have no visible cursor.

Answer 1

I’m now of the opinion that short of editing and then compiling my own build of Qt (not an option for me), I can’t force drag and drop operations to be internal only.

Equally, I can’t find any way of getting rid of the ghost trail by tweaking my window manager settings or using a compositing window manager (thanks anyway though @atomice). Using OpenGL as the renderer increases the screen repaint speed slightly, but is not perfect and introduces its own problems (see Starting a Qt drag operation on X11 w/ OpenGL causes screen flicker). I would still be very interested to hear any ideas though.

I have, however, got a workaround for my problem which works on both Windows and X. Here’s a simplified version:

void DoDrag()
{
  //Prepare the graphical representation of the drag
  mDragRepresenter = new QWidget(QApplication::activeWindow());
  mDragRepresenter->setAttribute(Qt::WA_TransparentForMouseEvents);
  mDragRepresenter->SetPixmap(GenerateDragPixmap());

  RepositionDragRepresenter();
  mDragRepresenter->show();

  QTimer UpdateTimer;
  connect(&UpdateTimer, SIGNAL(timeout()), this, SLOT(RepositionDragRepresenter()));
  UpdateTimer.start(40);

  //Start the drag (modal operation)
  Qt::DropAction ResultingAction = Drag->exec(Qt::CopyAction);

  UpdateTimer.stop();
  delete mDragRepresenter;
}

void RepositionDragRepresenter()
{
  mDragRepresenter->move(QApplication::activeWindow()->mapFromGlobal(QCursor::pos()) - mDragRepresenterHotSpot);
}