My app seems to be working now. I'm going to keep this unsolved for another day, just in case someone has something else to add.

So

I thought I'd already tried your suggestion to use disconnect() without success, but I tried it again, and now it seems to work.

void Widget::showFileTransfer()
{
    m_fileTransferDialog = new QProgressDialog("Transferring file", "Abort Transfer", 0, 100, this);
    QObject::connect(this, &Widget::progressChanged, m_fileTransferDialog, &QProgressDialog::setValue);
    QObject::connect(m_fileTransferDialog, &QProgressDialog::canceled, this, &Widget::relayTransferCanceled);
    m_fileTransferDialog->show();
}

void Widget::relayTransferCanceled()
{
    QObject::disconnect(m_fileTransferDialog, &QProgressDialog::canceled, nullptr, nullptr);
    emit transferCanceled();
    deleteProgressDialog(FILE_TRANSFER_CANCELED);
}

I'd like to get to the bottom of this issue, so I'm going to leave it as unsolved for a few days, but at least I have a workaround. Thanks.

I know nothing about this, but does closing the dialog cause the QProgressDialog::canceled signal to be raised? Does unconnecting that signal prior to m_fileTransferDialog->close(); solve your problem? take a look at https://www.qtcentre.org/threads/53056-QProgressDialog-cancel-signal-is-emitted-always ?

Also, you are calling deleteProgressDialog() in your relayTransferCanceled() slot. Should you/is it safe to be going

    m_fileTransferDialog->close();
    delete m_fileTransferDialog;

from a slot? Should it be more link deleteLater()?

These are just thoughts for you! I know nothing... :)

@JonB that's exactly what it does -- crash on the second close() attempt.

Aha! OK, you didn't say that :)

So I presume when you hit each time the stacktraces don't reveal anything different?

If several, you should reconsider your dialog and give it the capability to show information for more than one transfert.

In looking at the canceled() signal, I wonder if I should disconnect it from the default slot cancel() before I use it. I notice that closing the window also triggers the signal.

EDIT:

Tried the above, no help. I'm definitely doing something to cause this code to execute twice, but I sure don't know what it is. Here's the latest effort:

void Widget::showFileTransfer()
{
    m_fileTransferDialog = new QProgressDialog("Transferring file", "Abort Transfer", 0, 100, this);
    QObject::connect(this, &Widget::progressChanged, m_fileTransferDialog, &QProgressDialog::setValue);
    QObject::connect(m_fileTransferDialog, &QProgressDialog::canceled, this, &Widget::relayTransferCanceled);
    m_fileTransferDialog->show();
}

void Widget::relayTransferCanceled()
{
    emit transferCanceled();
    deleteProgressDialog(false);void Widget::deleteProgressDialog(bool success)
}
void Widget::deleteProgressDialog(bool success)
{
    QString title;
    QString text;

    if (success)
    {
        title = "File Transfer Complete";
        text = "The file transfer has completed.";
    }
    else
    {
        title = "File Transfer Did Not Complete";
        text = "The file transfer did not complete successfully.";
    }

    QMessageBox *qmb;

    qmb = new QMessageBox(QMessageBox::NoIcon,
                    title,
                    text,
                    QMessageBox::Ok,
                    this);
    qmb->exec();
    m_fileTransferDialog->close();
    delete m_fileTransferDialog;
    delete qmb;
}

m_fileTransferDialog

You only delete the QProgressDialog when it's canceled, at least in the show you've shown us.

I've deleted the build directory and rebuilt...same results.

The QDialog documentation doesn't really go into the difference between show() and open(), but they look almost identical in this use instance.

The latter is just show() for a modal dialog (i.e. it calls setModal(true) for you). I had that anti-epiphany due to QProgress's own open() overload, which allows you to bind a function pointer to one of the dialog's signals. Anyway.
Is it possible you have created 2 dialogs and due to having two objects emitting the signal you get to the slot twice? That'd be the most "obvious" thing I'd check as you're holding objects as members and it happens to us all (unfortunately) to forget having an object and just replacing the pointer member variable without realizing.

Disregard, I must've had a brain aneurysm or something.

I hadn't been destroying the object, but now I do (same results). Here's the slot:

void Widget::relayTransferCanceled()
{
    emit fwUpdateCanceled();
    m_fileTransferDialog->close();
    delete m_fileTransferDialog;
    QMessageBox qmb(QMessageBox::NoIcon,
                    "File Transfer Canceled",
                    "The file transfer has been canceled.",
                    QMessageBox::Ok,
                    this);
}

Does that code get called several times ?
Do you properly delete m_fileTransferDialog after use or before calling that part ?