A QDialog can't be exited on Windows, but not on MacOS/

I have a dialog in Qt5(C++). You can see that the dialog just creates some widgets (VisGLWidget from QOpenGLWidget) and keeps updating it.

#include "visualizationdlg.h"
#include "ui_visualizationdlg.h"

VisualizationDlg::VisualizationDlg(QWidget *parent, NewtonSpace *data) :
    QDialog(parent),
    ui(new Ui::VisualizationDlg)
{
    ui->setupUi(this);
    this->setFixedSize(600,600);

    this->data = data;
    this->visualizationGL = new VisGLWidget(this, this->data);
    this->visualizationGL->setObjectName(QString::fromUtf8("visualizationGL"));
    this->visualizationGL->setGeometry(QRect(10, 50, 581, 541));
    this->visualizationGL->repaint();

    this->rePaintTimer = startTimer(1);
}

VisualizationDlg::~VisualizationDlg()
{
    delete ui;
}

void VisualizationDlg::timerEvent(QTimerEvent *event)
{
    if(event->timerId() == this->rePaintTimer) {
        this->data->update(ui->GInput->value(), ui->updateSpdInput->value());
        this->visualizationGL->repaint();

        this->rePaintTimer = startTimer(1);
    }
}

The question is, I can quit this dialog by clicking the cross on MacOS, but not on Windows. What's wrong with it?

In your VisualizationDlg constructor you have...

this->rePaintTimer = startTimer(1);

That will result in a QTimerEvent being sent to your VisualizationDlg instance once every millisecond (potentially).

Now consider the code that handles timer events...

void VisualizationDlg::timerEvent (QTimerEvent *event)
{
    if(event->timerId() == this->rePaintTimer) {
        this->data->update(ui->GInput->value(), ui->updateSpdInput->value());
        this->visualizationGL->repaint();

        this->rePaintTimer = startTimer(1);
    }
}

The first time this is invoked the id associated with the timer event will match rePaintTimer. Hence repaint will be called. But you then start a new timer and assign its id to rePaintTimer without stopping the original timer. You now have two timers each generating events at 1KHz but your event handler only ever acts on events from the most recently created timer.

Now think what this will become after even a few seconds. You could potentially have thousands of timers each generating events at 1KHz.

Remove the line...

this->rePaintTimer = startTimer(1);

from your event handler to make it...

void VisualizationDlg::timerEvent (QTimerEvent *event)
{
    if(event->timerId() == this->rePaintTimer) {
        this->data->update(ui->GInput->value(), ui->updateSpdInput->value());
        this->visualizationGL->repaint();
    }
}