Layout don`t resize subWindow in mdiArea

layout doesn`t resize subwindow when tab was added. but when I resize subWindow, layout resized subWindow automatically (layout 'setMinimumSize' for subWindow)

#include "buglayoutsubwindow.h"
bugLayoutSubWindow::bugLayoutSubWindow(QWidget *parent, Qt::WFlags flags)
: QMainWindow(parent, flags)
{
ui.setupUi(this);

/create widget of subWindow/
m_widget = new QWidget(0);
/create any layout (vertical) in temp widget (of subWindow)/
QVBoxLayout * layout=new QVBoxLayout(m_widget);

/add widgets to layout/
m_tabWidget =new QTabWidget(m_widget);
layout->addWidget(m_tabWidget);
m_pushButton = new QPushButton("add tab", m_widget);
layout->addWidget(m_pushButton);

m_widget->setLayout(layout);
/set widget into subWindow/
ui.mdiArea->addSubWindow(m_widget);
/signal-slot for create new tab in subWindow/
connect(m_pushButton,SIGNAL(clicked()),this,SLOT(addTab()));
}
void bugLayoutSubWindow::addTab()
{
/create widget into 'new' tab/
QWidget *widget = new QWidget(m_tabWidget);

/create any layout (vertical) into 'new' tab's widget/
QVBoxLayout * layout=new QVBoxLayout(widget);

/add widgets to layout/
QPushButton *pushButton = new QPushButton("verify layout", widget);
layout->addWidget(pushButton);

widget->setLayout(layout);

/add new tab - set widget with layout/
m_tabWidget->addTab(widget,"new tab");

/* bug - layout don`t resize subwindow when tab was added. but when I resize subWindow, layout resized subWindow automatically (layout 'setMinimumSize' for subWindow)*/
}