How to move the whole window when mouse is on the window's custom widget in Qt?

Question

Let's say I have a custom widget and add it to the main window in qt.

As you can see, the red area is the custom widget. What I want to do is when the mouse is pressed in the red area and moved, the whole window will move as well.

I know how to simply implement mousePressEvent and mouseMoveEvent; but when dealing with a window with the custom widget, I do not know how to move the whole window when mouse is pressed on the custom widget.

Also I want to mention that I only want the window movable when mouse is pressed and moved in the red area, and when mouse is pressed and moved in the rest part of the main window area, nothing will happen.

This is what my CustomWidget class looks like:

CustomWidget::CustomWidget(QWidget *parent) : QWidget(parent)
{
    setFixedSize(50, 50);
    setStyleSheet("QWidget { background: red; }");
}

void CustomWidget::paintEvent(QPaintEvent *)
{
    QStyleOption opt;
    opt.init(this);
    QPainter painter(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &painter, this);
}

void CustomWidget::mousePressEvent(QMouseEvent *event)
{
    xCoord = event->x();
    yCoord = event->y();
}

void CustomWidget::mouseMoveEvent(QMouseEvent *event)
{
    move(event->globalX() - xCoord, event->globalY() - yCoord);
}

In case you wonder why I want to do this, in my app, I hid the title bar and drew a custom title bar by myself. But the window is not movable, so I want to make the whole window movable when mouse is pressed and moved on the title bar.
Hope I explained myself clearly.

Answer 1

To move the window from any widget it is necessary to be able to access the window, and for this we use the method window() that returns the top level, it is not necessary to separate the coordinates x() and y(), the following code implements the solution:

customwidget.h

#ifndef CUSTOMWIDGET_H
#define CUSTOMWIDGET_H

#include <QWidget>

class CustomWidget : public QWidget
{
    Q_OBJECT
public:
    explicit CustomWidget(QWidget *parent = nullptr);
protected:
    void paintEvent(QPaintEvent *);
    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
private:
    QPoint startPos;
};

#endif // CUSTOMWIDGET_H

customwidget.cpp

#include "customwidget.h"

#include <QMouseEvent>
#include <QPainter>
#include <QStyleOption>

CustomWidget::CustomWidget(QWidget *parent) : QWidget(parent)
{
    setFixedSize(50, 50);
    setStyleSheet("QWidget { background: red; }");
}

void CustomWidget::paintEvent(QPaintEvent *)
{
    QStyleOption opt;
    opt.init(this);
    QPainter painter(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &painter, this);
}

void CustomWidget::mousePressEvent(QMouseEvent *event)
{
    startPos = event->pos();
    QWidget::mousePressEvent(event);
}

void CustomWidget::mouseMoveEvent(QMouseEvent *event)
{
    QPoint delta = event->pos() - startPos;
    QWidget * w = window();
    if(w)
        w->move(w->pos() + delta);
    QWidget::mouseMoveEvent(event);
}