Cannot show modal dialog or block in QFileSystemModel::dropMimeData() or QTreeView::dropEvent() in Qt 6 during drag and drop

PuskasAlex

Description:

In Qt 5.15 it was possible to show a modal dialog (e.g., QMessageBox::question()) directly inside QFileSystemModel::dropMimeData() or in QTreeView::dropEvent() during a drag and drop operation to ask the user for confirmation before accepting the drop.

Since upgrading to Qt 6 (tested on Qt 6.8), attempting to show a modal dialog or block the event processing in these functions results in an application crash. This breaks the workflow for applications that need explicit user confirmation before accepting a drop.

Steps to reproduce:

Override QTreeView::dropEvent().
In this override, display a modal QMessageBox::question() dialog to confirm the drop.
Run the application.
Perform a drag and drop operation.

Expected behavior:

The dialog should appear.
Depending on the user response, the drop is accepted or rejected.

Actual behavior:

The application crashes immediately when the dialog is shown during dropEvent().

Code snippet:

class MyTreeView : public QTreeView {
public:
    using QTreeView::QTreeView;

protected:
    void dropEvent(QDropEvent* event) override {
        if (event->mimeData()->hasUrls()) {
            if (QMessageBox::question(this, "title", "body") == QMessageBox::Yes) {
                QTreeView::dropEvent(event);
            } else {
                event->ignore();
            }
        } else {
            event->ignore();
        }
    }
};

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

    QFileSystemModel *model = new QFileSystemModel();
    model->setRootPath(QDir::homePath());
    model->setReadOnly(false);

    MyTreeView *view = new MyTreeView();
    view->setModel(model);
    view->setRootIndex(model->index(QDir::homePath()));
    view->setDragEnabled(true);
    view->setAcceptDrops(true);
    view->setDropIndicatorShown(true);
    view->setWindowTitle("Drop Test Qt 6");
    view->resize(800, 600);
    view->show();

    return app.exec();
}

This application crashes using Qt 6.8 but not using Qt 5.15.2 on Windows.

Environment:

Qt Version: 6.8 (also occurs on 6.5)
OS: Windows 10 x32
Compiler: MSVC 2022

Questions:

Do you have any recommendation or workaround for this issue?
How can this issue be avoided in Qt 6?

Zbigniew-Sch

@PuskasAlex said in Cannot show modal dialog or block in QFileSystemModel::dropMimeData() or QTreeView::dropEvent() in Qt 6 during drag and drop:

QTreeView::dropEvent(event);

Hi,

Delete QTreeView::dropEvent(event) and everything will work fine:

if (QMessageBox::question(this, "title", "body") == QMessageBox::Yes)
event->accept();;
else
event->ignore();