Bug(?) with modal dialog and drag&dop

tgoetz

Hi Qt hive,

I have a user interface with dialogs. In one of the dialogs I am using QListWidgets to present QListWidgetItems that the user can drag&drop from one list to other receiver list.

The source code of my dialog looks like this:

[...]
DragDialog::DragDialog(QWidget* parent) : QDialog(parent)
{
  setWindowTitle("Drag&Drop Test");
  QListWidget* source = new QListWidget();
  source->setAlternatingRowColors(true);
  source->setSelectionMode(QAbstractItemView::SingleSelection);
  source->setDragEnabled(true);
  source->setDragDropMode(QAbstractItemView::DragDrop);
  source->setDefaultDropAction(Qt::MoveAction);
  source->viewport()->setAcceptDrops(true);
  source->setDropIndicatorShown(true);

  // names is a QStringList with the names of US presidents just to fill the list
  for(auto const name : names)
  {
    auto item = new QListWidgetItem(name);
    item->setToolTip(name);
    item->setData(Qt::UserRole, name);
    source->addItem(item);
  }

  QListWidget* target1 = new QListWidget();
  target1->setAlternatingRowColors(true);
  target1->setSelectionMode(QAbstractItemView::SingleSelection);
  target1->setDragEnabled(true);
  target1->setDragDropMode(QAbstractItemView::DragDrop);
  target1->setDefaultDropAction(Qt::MoveAction);
  target1->viewport()->setAcceptDrops(true);
  target1->setDropIndicatorShown(true);

  QHBoxLayout* layout = new QHBoxLayout();
  layout->addWidget(source);
  layout->addWidget(target);

  setLayout(layout);
}
[...]

And here is the relevant part of the corresponding main.cpp :

int main(int argc, char* argv)
{
  QApplication app(argc, argv);

  DragDialog dialog;
  dialog.show();

  app.connect(&app, &QApplication::lastWindowClosed, &app, &QApplication::quit);

  return app.exec();  
}

When I am using show() - as in the main.cpp above - to display the dialog, everything works fine and I can drag items from one list to the other and back. However, if I use exec(), the dialog freezes when I start to move a single item and I have to log in from a different machine to kill the process.

Has anybody seen this before and a way out?

I am using a system installation of Qt 5.9.2 on CentOS 7.5 (1804). On a different system, we have been using 5.6 and 5.9.3 and it was no problem.

Thanks!

tgoetz

I was able to reduce the code to this snippet that still generates the error on my system:

#include <QtWidgets/QApplication>
#include <QtWidgets/QDialog>
#include <QtWidgets/QListWidgetItem>
#include <QtWidgets/QListWidget>
#include <QtWidgets/QVBoxLayout>


int main (int argc, char **argv)
{
  QApplication app(argc, argv);

  QDialog dialog;

  QListWidget* source = new QListWidget();
  source->setDragEnabled(true);

  new QListWidgetItem("foo", source);

  new QVBoxLayout(&dialog);
  dialog.layout()->addWidget (source);

  return dialog.exec();
}

tgoetz

This is an actual bug in 5.9.2, see https://bugreports.qt.io/browse/QTBUG-63968