Qt. Problem z rysowaniem w viewport

#ifndef GUIBOARD_HPP
#define GUIBOARD_HPP

#include <QWidget>
#include <QRect>

class GuiBoard : public QWidget {
public:
    explicit GuiBoard(QWidget* parent = nullptr);

protected:
    void paintEvent(QPaintEvent* event) override;
    void resizeEvent(QResizeEvent* event) override;

private:
    QRect m_viewport;
};

#endif  // GUIBOARD_HPP

#include "GuiBoard.hpp"

#include <algorithm>    // min, max
#include <QDebug>
#include <QPainter>
#include <QResizeEvent>

GuiBoard::GuiBoard(QWidget* const parent) : QWidget(parent) {}

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

    painter.setViewport(m_viewport);
    qDebug() << "Current viewport is:" << painter.viewport();

    painter.fillRect(0, 0, painter.viewport().width(), painter.viewport().height(), Qt::red);
    painter.drawLine(0, 0, 50, 50);
}

void GuiBoard::resizeEvent(QResizeEvent* event) {
    const auto& size = event->size();
    qDebug() << "============\nnew size is:" << size;
    const int width = size.width();
    const int height = size.height();

    const int min = std::min(width, height);
    const int max = std::max(width, height);
    const int p = (max - min) / 2;  // top-left

    if(width > height)
        m_viewport.setRect(p, 0, min, min);
    else
        m_viewport.setRect(0, p, min, min);
}

============
new size is: QSize(640x246)
Current viewport is: QRect(197,0 246x246)

void GuiBoard::paintEvent(QPaintEvent* const event) {
    QPainter painter(this);
    const auto& topLeft = m_viewport.topLeft();

    painter.fillRect(m_viewport, Qt::red);

    const int x1 = 20;  // hardcoded
    const int y1 = 20;  // hardcoded
    const int x2 = 50;  // hardcoded
    const int y2 = 90;  // hardcoded
    painter.drawLine(QPoint{x1, y1} + topLeft,
                     QPoint{x2, y2} + topLeft);
}

void GuiBoard::mousePressEvent(QMouseEvent* event) {
    qDebug() << event->pos() - m_viewport.topLeft();
}