QBoxLayout with QMainWindow vs QWidget

Hello.I'd like to ask about the thing that is happening ,when I use QHBoxLayout with QMainWindow and with QWidget.I'm trying to change window title style, by connecting 2 widgets.
This what's happening: Under QWidget
Attachment 10269
Here's the code for class under QWidget:
TitleBar.h

Code:

#ifndef MYTITLEBAR_H
#define MYTITLEBAR_H
 
#include <QMainWindow>
#include <QMouseEvent>
#include <QToolButton>
#include <QStyle>
#include <QLabel>
#include <QHBoxLayout>
#include <QPixmap>
 
class MyTitleBar : public QWidget
{
public:
    MyTitleBar( QWidget* parent );
public slots:
    void showSmall();
    void showMaxRestore();
 
protected:
    void mousePressEvent( QMouseEvent* me );
    void mouseMoveEvent( QMouseEvent* me );
 
private:
    QToolButton* minimize;
    QToolButton* maximize;
    QToolButton* close;
    QToolButton* custom;
    QPixmap restorePix, maxPix, closePix, minPix, customPix;
    bool maxNormal;
    QPoint startPos;
    QPoint clickPos;
};
 
#endif // MYTITLEBAR_H

TitleBar.cpp

Code:

#include "mytitlebar.h"
 
MyTitleBar::MyTitleBar( QWidget* parent )
{
    this->setWindowFlags( Qt::FramelessWindowHint );
 
    minimize = new QToolButton(this);
    maximize = new QToolButton(this);
    close = new QToolButton(this);
    custom = new QToolButton(this);
 
    closePix.load( "close.png" );
    close->setIcon( closePix );
 
    maxPix.load( "maximize.png" );
    maximize->setIcon( maxPix );
 
    minPix.load( "minimize.png" );
    minimize->setIcon( minPix );
 
    customPix.load( "custom_icon.png" );
    custom->setIcon( customPix );
 
    QLabel* label = new QLabel(this);
    label->setText( "Custom Window" );
 
    QHBoxLayout* HBox = new QHBoxLayout( this );
 
    HBox->addWidget( custom );
    HBox->addWidget( label );
    HBox->addWidget( minimize );
    HBox->addWidget( maximize );
    HBox->addWidget( close );
 
 
 
}
 
void MyTitleBar::showSmall()
{
 
}
 
void MyTitleBar::showMaxRestore()
{
 
}
 
void MyTitleBar::mousePressEvent(QMouseEvent *me)
{
 
}
 
void MyTitleBar::mouseMoveEvent(QMouseEvent *me)
{
 
}

But under QMainWindow, happens something like this:
Attachment 10270

The code is exactly the same as for QWidget, but I've changed the parent and inheritance to QMainWindow.And returning to the question why is this happening and how I can fix this? In the main window it's just intancing the class and move it to the right place, that it'd look like a window title bar and is the same as for QWidget and QMainWindow. I need to use QMainWindow as parent, because the main window is inheritance of QMainWindow.