QT: Почему мои виджеты "трясутся" при изменении размера окна?

Question

In my UI design, I want four QPushButtons stay on the right side of window. So I create a HBoxLayout for the centralWidget, and add a QWidget and a VBoxLayout(which contains the four buttons) to it. The stretch factor for the VBoxLayout is set to 0, so that it will stays stay on the right side with a minimum width.

However, when I try to resize the window from the left, my QPushButtons kind of "shakes" during resizing, which brings a very bad looking.

So what might be the proper reason for this? Is there any solution that such "shaking" can be avoided?

A reproducible example:

mainwindow.ui:

<?xml version="1.0" encoding="UTF-8"?>
 MainWindow   0  0  800  600    MainWindow      Кнопка            0  0  800  23       

Или сгенерированный ui_mainwindow.h файл:

/********************************************************************************
** Form generated from reading UI file 'mainwindow.ui'
**
** Created by: Qt User Interface Compiler version 5.15.0
**
** WARNING! All changes made in this file will be lost when recompiling UI file!
********************************************************************************/

#ifndef UI_MAINWINDOW_H
#define UI_MAINWINDOW_H

#include <QtCore/QVariant>
#include <QtWidgets/QApplication>
#include <QtWidgets/QHBoxLayout>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QMenuBar>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QStatusBar>
#include <QtWidgets/QVBoxLayout>
#include <QtWidgets/QWidget>

QT_BEGIN_NAMESPACE

class Ui_MainWindow
{
public:
    QWidget *centralwidget;
    QHBoxLayout *horizontalLayout_2;
    QHBoxLayout *horizontalLayout;
    QWidget *widget;
    QVBoxLayout *verticalLayout_2;
    QPushButton *pushButton;
    QMenuBar *menubar;
    QStatusBar *statusbar;

    void setupUi(QMainWindow *MainWindow)
    {
        if (MainWindow->objectName().isEmpty())
            MainWindow->setObjectName(QString::fromUtf8("MainWindow"));
        MainWindow->resize(800, 600);
        centralwidget = new QWidget(MainWindow);
        centralwidget->setObjectName(QString::fromUtf8("centralwidget"));
        horizontalLayout_2 = new QHBoxLayout(centralwidget);
        horizontalLayout_2->setObjectName(QString::fromUtf8("horizontalLayout_2"));
        horizontalLayout = new QHBoxLayout();
        horizontalLayout->setObjectName(QString::fromUtf8("horizontalLayout"));
        widget = new QWidget(centralwidget);
        widget->setObjectName(QString::fromUtf8("widget"));

        horizontalLayout->addWidget(widget);

        verticalLayout_2 = new QVBoxLayout();
        verticalLayout_2->setObjectName(QString::fromUtf8("verticalLayout_2"));
        pushButton = new QPushButton(centralwidget);
        pushButton->setObjectName(QString::fromUtf8("pushButton"));

        verticalLayout_2->addWidget(pushButton);


        horizontalLayout->addLayout(verticalLayout_2);

        horizontalLayout->setStretch(0, 1);

        horizontalLayout_2->addLayout(horizontalLayout);

        MainWindow->setCentralWidget(centralwidget);
        menubar = new QMenuBar(MainWindow);
        menubar->setObjectName(QString::fromUtf8("menubar"));
        menubar->setGeometry(QRect(0, 0, 800, 23));
        MainWindow->setMenuBar(menubar);
        statusbar = new QStatusBar(MainWindow);
        statusbar->setObjectName(QString::fromUtf8("statusbar"));
        MainWindow->setStatusBar(statusbar);

        retranslateUi(MainWindow);

        QMetaObject::connectSlotsByName(MainWindow);
    } // setupUi

    void retranslateUi(QMainWindow *MainWindow)
    {
        MainWindow->setWindowTitle(QCoreApplication::translate("MainWindow", "MainWindow", nullptr));
        pushButton->setText(QCoreApplication::translate("MainWindow", "PushButton", nullptr));
    } // retranslateUi

};

namespace Ui {
    class MainWindow: public Ui_MainWindow {};
} // namespace Ui

QT_END_NAMESPACE

#endif // UI_MAINWINDOW_H

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"
#include "./ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete 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();
}