How to make a modal dialog stay on top of an existing modal dialog?

[schrute]

Hi,
I need to create a modal dialog on top of another modal dialog. Following code shows how I am creating the dialogs.

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{    
    auto mainWidget = new QWidget(this);    
    auto button = new QPushButton("Click", mainWidget);

    QVBoxLayout* layout = new QVBoxLayout;
    layout->addWidget(button, Qt::AlignCenter);
    
    mainWidget->setLayout(layout);
    
    setCentralWidget(mainWidget);

    connect(button, SIGNAL(clicked()), this, SLOT(buttonClick()));
}

void MainWindow::buttonClick1() {
    m_dialog2 = new QDialog(m_dialog1);
    m_dialog2->exec();
}

void MainWindow::buttonClick()
{
    m_dialog1 = new QDialog(this);
    auto layout = new QVBoxLayout;

    auto button = new QPushButton("Click", m_dialog1);
    connect(button, SIGNAL(clicked()), this, SLOT(buttonClick1()));

    layout->addWidget(button);

    m_dialog1->setLayout(layout);
    m_dialog1->exec();
}

The problem is that the second dialog (m_dialog2) is modal with respect to the main window, but loses focus and moves to the background if the first dialog is clicked (m_dialog1). Although the first dialog accepts focus, it doesn't accept any click events, which makes it feel like the app is frozen.
How can I make the second dialog modal with respect to the first dialog as well?

Thanks!

EDIT: I am testing this on MacOS 10.14.6/Qt 5.12.4.

[schrute]

This turned out to be a bug in Qt 5.12.4 and was fixed on upgrading to 5.15.

[SGaist]

Hi,

AFAIK, you did set the modality correctly however you are chaining blocking event loops when calling exec and IIRC, you can have issues with that. I would recommend moving to the use of QDialog::open which changes how you handle the dialog slightly but avoids nested event loops.

[schrute]

@SGaist Unfortunately I am working on a legacy codebase, so I would like to avoid changing the QDialog::exec calls to QDialog::open (to avoid code restructuring at this point).

Though I did change exec to open in the sample code that I shared. It does work but converts both the dialogs to sheets even though I am not specifying Qt::Sheet window flag. In fact, if I explicitly remove the Qt::Sheet window flag from the dialog (after calling open), it disappears/closes.

I also tried changing the window modality to Qt::WindowModal while using exec, but it also presents the dialogs as sheets.

[JonB]

@schrute

The problem is that the second dialog (m_dialog2) is modal with respect to the main window, but loses focus and moves to the background if the first dialog is clicked (m_dialog1).

Does it? It doesn't help that you didn't think it relevant to state what platform you are on. I am Qt 5.12.2 under Ubuntu 19.04, GNOME desktop. All works as expected. m_dialog1 is modal wrt MainWindow, m_dialog2 is modal wrt m_dialog1 (not sure what you meant by your "The problem is that the second dialog (m_dialog2) is modal with respect to the main window,", it isn't, unless you mean "indirectly" via grand-parentage), and in all cases the correct dialog remains up-front and no back-dalog can be clicked/focussed while it's there.....

I believe, but admit I am not sure, that in the past I had an application which did this under Windows too, and the behaviour was the same as under Linux, with the modal hierarchy being perfectly respected.

Furthermore, the default for QDialog::exec() is already Qt::WindowModal for a dialog with a parent [EDIT incorrect, as per @schrute reply below], so setting m_dialog[12]->setWindowModality(Qt::WindowModal) had no different effect, and I did not get any "but it also presents the dialogs as sheets". Nor did I find changing exec() to open() caused any "sheet-ness".

[schrute]

@JonB

It doesn't help that you didn't think it relevant to state what platform you are on.

I am testing this on MacOS 10.14.6 with Qt 5.12.4.

Furthermore, the default for QDialog::exec() is already Qt::WindowModal for a dialog with a parent

As per the documentation, by default the dialog is Qt::ApplicationModal.

[JonB]

@schrute said in How to make a modal dialog stay on top of an existing modal dialog?:

I am testing this on MacOS 10.14.6 with Qt 5.12.4.

Maybe that is the difference in behaviour, I cannot say.

As per the documentation, by default, the dialog is Qt::ApplicationModal.

Damn, you are right! I could have sworn that the default was application modal when parent = nullptr but window modal when parent = something.... So for a QDialog the relevance of parent is just

but if it has a parent, its default location is centered on top of the parent

Right?

[schrute]

This turned out to be a bug in Qt 5.12.4 and was fixed on upgrading to 5.15.