When a QToolBar is not a child of a QMainWindow, it loses the ability to populate the extension pop up with widgets added to the toolbar using addWidget(). Please use widget actions created by inheriting QWidgetAction and implementing QWidgetAction::createWidget() instead.

I tried to implement my subclass as

#ifndef TOOLBUTTONACTION_H
#define TOOLBUTTONACTION_H

#include <QMenu>
#include <QPointer>
#include <QToolButton>
#include <QWidgetAction>

class ToolButtonAction : public QWidgetAction
{
    Q_OBJECT
  public:
    explicit ToolButtonAction( QObject *parent = nullptr );
    void setAutoRaise( bool autoRaise );
    void setPopupMode( QToolButton::ToolButtonPopupMode mode );
    void setToolButtonStyle( Qt::ToolButtonStyle style );
    void setMenu( QMenu *menu );
    void addActions( const QList<QAction *> &actions );
    void setDefaultAction( QAction *action );
    QAction *defaultAction() const;

  protected:
    QWidget *createWidget( QWidget *parent ) override;
    void deleteWidget( QWidget *widget ) override;

  private:
    void applyState( QToolButton *button ) const;

  private:
    bool mAutoRaise = false;
    Qt::ToolButtonStyle mToolButtonStyle = Qt::ToolButtonStyle::ToolButtonIconOnly;
    QToolButton::ToolButtonPopupMode mPopupMode = QToolButton::ToolButtonPopupMode::DelayedPopup;
    QPointer< QMenu > mMenu;
    QList< QAction * > mButtonActions;
    QPointer< QAction > mDefaultAction;
    QPointer< QToolButton > mButton;
};

#endif // TOOLBUTTONACTION_H

and

#include "toolbuttonaction.h"

#include <QMenu>
#include <QToolBar>

ToolButtonAction::ToolButtonAction( QObject *parent )
  : QWidgetAction( parent )
{
}

void ToolButtonAction::setAutoRaise( bool autoRaise )
{
  mAutoRaise = autoRaise;
  if ( mButton )
  {
    mButton->setAutoRaise( autoRaise );
  }
}

void ToolButtonAction::setPopupMode( QToolButton::ToolButtonPopupMode mode )
{
  mPopupMode = mode;
  if ( mButton )
  {
    mButton->setPopupMode( mode );
  }
}

void ToolButtonAction::setToolButtonStyle( Qt::ToolButtonStyle style )
{
  mToolButtonStyle = style;
  if ( mButton )
  {
    mButton->setToolButtonStyle( style );
  }
}

void ToolButtonAction::setMenu( QMenu *menu )
{
  mMenu = menu;
  if ( mButton )
  {
    mButton->setMenu( menu );
  }
}

void ToolButtonAction::addActions( const QList< QAction * > &actions )
{
  for ( QAction *action : std::as_const( actions ) )
  {
    mButtonActions.append( action );
  }

  if ( mButton )
  {
    mButton->addActions( actions );
  }
}

void ToolButtonAction::setDefaultAction( QAction *action )
{
  if ( !action )
  {
    return;
  }

  mDefaultAction = action;

  if ( mButton )
  {
    mButton->setDefaultAction( action );
  }
}

QAction *ToolButtonAction::defaultAction() const
{
  return mDefaultAction;
}

QWidget *ToolButtonAction::createWidget( QWidget *parent )
{
  QToolButton *button = new QToolButton( parent );
  button->setAutoRaise( mAutoRaise );
  button->setPopupMode( mPopupMode );
  button->setToolButtonStyle( mToolButtonStyle );

  if ( QToolBar *toolBar = qobject_cast< QToolBar * >( parent ) )
  {
    button->setIconSize( toolBar->iconSize() );
    connect( toolBar, &QToolBar::iconSizeChanged, button, &QToolButton::setIconSize );
    button->setToolButtonStyle( toolBar->toolButtonStyle() );
    connect( toolBar, &QToolBar::toolButtonStyleChanged, button, &QToolButton::setToolButtonStyle );
  }

  if ( mMenu )
  {
    button->setMenu( mMenu );
  }

  if ( !mButtonActions.isEmpty() )
  {
    button->addActions( mButtonActions );
  }

  if ( mDefaultAction )
  {
    button->setDefaultAction( mDefaultAction );
  }
  else
  {
    applyState( button );
    connect( button, &QToolButton::clicked, this, &QAction::trigger );
  }

  mButton = button;
  return button;
}

void ToolButtonAction::deleteWidget( QWidget *widget )
{
  QWidgetAction::deleteWidget( widget );
}

void ToolButtonAction::applyState( QToolButton *button ) const
{
  button->setEnabled( isEnabled() );

  if ( !mDefaultAction )
  {
    button->setIcon( icon() );
    button->setText( text() );
    button->setToolTip( toolTip() );
  }
}

This works, but I'm not sure I correctly understand and handle case when the same button/action is added in different places, e.g. to the main toolbar/menu and secondary toolbar. Should I keep track of created buttons in my subclass, e.g. to change their properties like autoraise/popup mode/etc. or returning a single widget is enough? My understanding is that createWidget() is called once per container to which I add action. So when for example I add action to a toolbar and then shrink it, a new action is also created for extension popup and I will end up with two actions. Am right?

If my understanding is correct, I will need to replace QPointer< QToolButton > mButton; with QList< QPointer< QToolButton > > mButtons; and then in all setters do something like

  for ( QToolButton *button : std::as_const( mButtons ) )
  {
    if ( button )
    {
      button->setAutoRaise( autoRaise );
    }
  }