Changing dialog's parent disables drag and drop - c++

Question

I have a main window and a dialog that is opened from this window on a button click. For performance reasons, there is a dialog cache, that keeps an instance of the dialog and only shows it when the dialog should be opened instead of creating new instance. In the dialog, there's a QListWidget with some items which order can be changed by drag and drop. This works when I first open the dialog, but when I close it and open it again, I'm unable to drop the items, I get a Qt::ForbiddenCursor.
The issue seems to be caused by calling setParent(nullptr) when closing the dialog (or likely by just changing the parent). If I remove this line, drag and drop works. However I need this to prevent the dialog from being deleted by the parent and also the dialog can have different parents in different contexts (this isn't obvious from my simplified example). Any idea what is wrong with this approach? My Qt version is 5.9.3. Can this be a Qt bug?
MainWindow.h:
#include "ui_mainwindow.h"
#include "dialog.h"
#include <QPushButton>
#include <QMainWindow>
#include <memory>
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget* parent = nullptr) : QMainWindow(parent), ui(new Ui::MainWindow)
{
ui->setupUi(this);
dialog.reset(new Dialog(this));
dialog->setAttribute(Qt::WA_DeleteOnClose, false);
connect(ui->button, &QPushButton::pressed, [&]
{
dialog->setParent(this, dialog->windowFlags());
dialog->open();
});
}
~MainWindow()
{
delete ui;
}
private:
Ui::MainWindow* ui;
std::unique_ptr<Dialog> dialog;
};
Dialog.h:
#include "ui_dialog.h"
#include <QDialog>
class Dialog : public QDialog
{
Q_OBJECT
public:
explicit Dialog(QWidget* parent) : QDialog(parent), ui(new Ui::Dialog)
{
ui->setupUi(this);
ui->listWidget->addItem("first");
ui->listWidget->addItem("second");
ui->listWidget->addItem("third");
}
~Dialog()
{
delete ui;
}
public slots:
virtual void reject() override
{
setParent(nullptr);
QDialog::reject();
}
private:
Ui::Dialog* ui;
};
Dialog.ui - simple dialog with QListWidget and reject button
<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
<class>Dialog</class>
<widget class="QDialog" name="Dialog">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>548</width>
<height>397</height>
</rect>
</property>
<property name="windowTitle">
<string>Dialog</string>
</property>
<layout class="QVBoxLayout" name="verticalLayout">
<item>
<widget class="QListWidget" name="listWidget">
<property name="dragDropMode">
<enum>QAbstractItemView::DragDrop</enum>
</property>
<property name="defaultDropAction">
<enum>Qt::MoveAction</enum>
</property>
</widget>
</item>
<item>
<widget class="QDialogButtonBox" name="buttonBox">
<property name="orientation">
<enum>Qt::Horizontal</enum>
</property>
<property name="standardButtons">
<set>QDialogButtonBox::Close</set>
</property>
</widget>
</item>
</layout>
</widget>
<resources/>
<connections>
<connection>
<sender>buttonBox</sender>
<signal>accepted()</signal>
<receiver>Dialog</receiver>
<slot>accept()</slot>
<hints>
<hint type="sourcelabel">
<x>248</x>
<y>254</y>
</hint>
<hint type="destinationlabel">
<x>157</x>
<y>274</y>
</hint>
</hints>
</connection>
<connection>
<sender>buttonBox</sender>
<signal>rejected()</signal>
<receiver>Dialog</receiver>
<slot>reject()</slot>
<hints>
<hint type="sourcelabel">
<x>316</x>
<y>260</y>
</hint>
<hint type="destinationlabel">
<x>286</x>
<y>274</y>
</hint>
</hints>
</connection>
</connections>
</ui>
MainWindow.ui - default main window with one button
<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
<class>MainWindow</class>
<widget class="QMainWindow" name="MainWindow">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>432</width>
<height>316</height>
</rect>
</property>
<property name="windowTitle">
<string>MainWindow</string>
</property>
<widget class="QWidget" name="centralWidget">
<widget class="QPushButton" name="button">
<property name="geometry">
<rect>
<x>40</x>
<y>30</y>
<width>80</width>
<height>21</height>
</rect>
</property>
<property name="text">
<string>PushButton</string>
</property>
</widget>
</widget>
<widget class="QMenuBar" name="menuBar">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>432</width>
<height>20</height>
</rect>
</property>
</widget>
<widget class="QToolBar" name="mainToolBar">
<attribute name="toolBarArea">
<enum>TopToolBarArea</enum>
</attribute>
<attribute name="toolBarBreak">
<bool>false</bool>
</attribute>
</widget>
<widget class="QStatusBar" name="statusBar"/>
</widget>
<layoutdefault spacing="6" margin="11"/>
<resources/>
<connections/>
</ui>
main.cpp
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}

Answer 1

The below reproduces the issue. It is indeed a Qt bug. OP reported the bug: https://bugreports.qt.io/browse/QTBUG-70240
The problem is because QWidget recreates the drop site while the Qt::Window flag is off, invoking QWindowsWindow::updateDropSite, which does the wrong thing and calls setDropSiteEnabled(false).
The two equivalent workarounds are:
dialog->setParent(newParent) is replaced by:
auto flags = dialog->windowFlags();
dialog->setParent(newParent, {});
dialog->setWindowFlags(flags);
dialog->setParent(nullptr) is replaced by:
dialog->setParent(nullptr, dialog->windowFlags());
The first workaround undoes the damaged state of the widget. The second workaround doesn't, i.e. needs to be always used, or else the first workaround has to be invoked once to restore the usable drop target state.
// https://github.com/KubaO/stackoverflown/tree/master/questions/dialog-parent-dnd-52061919
#include <QtWidgets>
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QWidget ui;
QVBoxLayout layout{&ui};
QPushButton button{"Toggle List"};
QCheckBox workaround1{"Workaround 1"};
QCheckBox workaround2{"Workaround 2"};
for (auto w : QWidgetList{&button, &workaround1, &workaround2}) layout.addWidget(w);
workaround2.setChecked(true);
QListWidget listWidget;
Q_ASSERT(!listWidget.testAttribute(Qt::WA_DeleteOnClose));
listWidget.setWindowFlags(Qt::Window | Qt::FramelessWindowHint);
listWidget.setDragDropMode(QAbstractItemView::DragDrop);
listWidget.setDefaultDropAction(Qt::MoveAction);
for (auto s : QStringList{"first", "second", "third"}) listWidget.addItem(s);
QObject::connect(&button, &QPushButton::pressed, [&] {
if (!listWidget.parent()) {
if (!workaround1.isChecked())
listWidget.setParent(&button, listWidget.windowFlags());
else {
auto flags = listWidget.windowFlags();
listWidget.setParent(&button, {});
listWidget.setWindowFlags(flags);
}
listWidget.show();
} else {
if (!workaround2.isChecked())
listWidget.setParent(nullptr);
else
listWidget.setParent(nullptr, listWidget.windowFlags());
listWidget.close();
}
});
ui.setMinimumSize(320, 200);
ui.show();
return app.exec();
}