Weirdest behavior of a modal window when the parent is in move/size loop.

[nikitablack]

Greetings.

Please take a look at the following code:

// MainWindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "MainWindow.h"
#include "ui_mainwindow.h"
#include <QTimer>
#include <QErrorMessage>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0) : QMainWindow(parent), ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
        QTimer::singleShot(2000, this, &MainWindow::showChildWindow);
    }

    ~MainWindow()
    {
        delete ui;
    }
private:
    Ui::MainWindow *ui;

    void showChildWindow()
    {
        QErrorMessage* err{new QErrorMessage{this}};
        err->setWindowModality(Qt::ApplicationModal);
        err->showMessage("Test");
        err->setGeometry(100, 100, 300, 300);
        err->exec();
    }
};

#endif // MAINWINDOW_H

At the first look it seems ok. It's even works ok until you try to move/resize it before QErrorMessage appear. If this message was showed during such loop (move/size modal loop) then this message becomes disabled for moving/resizing also. Moreover, after closing it the parent window jumps to the previos position what looks jerky.

[nikitablack]

I tried to do the same with native Windows windows and it looks like that operating system works this way. Native modal windows are also inactive for dragging/resizing if was created during parent move/resize.