Unsolved drawEllipse() draws too many

[TomNow99]

Hi,

I have QScrollArea and QWidget in this area. That QWidget has QGridLayout and in this grid I have many QButtons in the first column ( I use only this one column, so I can use Vertical Layout rather than QGridLayout ). Now I would like to draw Circles on this QWidget. So in this QWidget I have paintEvent() function like this:

        firstButtonPositionY=buttons.first()->geometry().y()+buttons.first()->height()/2;
        lastButtonPositionY = buttons.last()->geometry().y()+buttons.last()->height()/2;

I get proper values.

Next I do:

    while( firstButtonPositionY < lastButtonPositionY )
    {
        firstButtonPositionY+=100;
        painter.drawEllipse(QPointF(width()/2, firstButtonPositionY) , 5, 5);
    }

I get too many circles:

I would like to have only green circles, but I get that red circles too ( in the picture I draw 2 rects: the first and the last buttons ).

When I change drawElippse to drawRect() or drawPoint() I get what I want.

    painter.drawRect(width()/2-5, firstButtonPositionY , 10, 30);

 or 

   painter.drawPoint(width()/2,firstButtonPositionY);

I get rects / points only between buttons, not after last Button or before the first Button.

So what I do wrong with this drawEllipse()?

EDIT: that problem is when I have many QButtons ( 500+ ).

EDIT2: The problem is when QWidget has height = 65640. For height = 65490 everything is ok. Pow(2,16) = 65536, so this is very strange.

EDIT3: When I change my code to:

    while( firstButtonPositionY < 100 )       // lastButtonPositionY is for example 300000
    {
        firstButtonPositionY+=1;
        painter.drawEllipse(QPointF(width()/2, firstButtonPositionY) , 5, 5);
    }

I get 100 long lines in 5 different places. So it looks like my QWidget is end on 2^16 height and started next. Next Widget started on 2 * 2^16, next 3* 2^16, next 4* 2^16.

EDIT 4

Please check that code:

mainWindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QScrollArea>
#include "mywidget.h"
#include <QGridLayout>

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;
    QScrollArea *area;
    myWidget *widget;
    QGridLayout *grid;
};
#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);
    area = new QScrollArea;
    widget = new myWidget;
    grid = new QGridLayout;

    centralWidget()->setLayout(grid);

    grid->addWidget(area);
    area->setWidget(widget);
}

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

myWidget.h

#ifndef MYWIDGET_H
#define MYWIDGET_H
#include <QWidget>
#include <QPaintEvent>
#include <QPushButton>
#include <QGridLayout>
class myWidget: public QWidget
{
    Q_OBJECT
    void paintEvent(QPaintEvent *event);
    QGridLayout *grid;
    QVector <QPushButton *> buttons;
public:
    myWidget(QWidget *parent = nullptr);
};

#endif // MYWIDGET_H

myWidget.cpp

#include "mywidget.h"
#include <QPainter>
myWidget::myWidget(QWidget *parent): QWidget(parent)
{
    grid = new QGridLayout;
    setLayout(grid);

    for(int i=0;i<1000;i++)
    {
        buttons.append(new QPushButton);
        buttons.last()->setFixedSize(50,50);
        grid->addWidget(buttons.last(),i,0);
    }
    grid->setSpacing(100);
    resize(minimumSizeHint());
}

void myWidget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);

    int begin = buttons.first()->y()+ buttons.first()->height();
    
    int xButtons = buttons.first()->x()+buttons.first()->width()/2;

    for(int i=begin; i<300;i++)            
    {
        painter.drawEllipse(QPointF(xButtons, i),1,1);
    }

    QWidget::paintEvent(event);
}

And pictures:

I would like to draw a few circles only at the beginning of myWidget

    for(int i=begin; i<300;i++)            
    {
        painter.drawEllipse(QPointF(xButtons, i),1,1);
    }

But there are circles in 3 places ( look at the picture with red and green circles ).

[TomNow99]

I add the fourth EDIT - I think the most important