main.cpp

#include <QMainWindow>
#include <QApplication>
#include <QMenu>
#include <QMenuBar>
#include <QWidgetAction>
#include <QTextEdit>
#include <QLabel>
#include <QDebug>

class MyActionWidget : public QLabel
{
public:
    MyActionWidget(const QString& text, QWidget* parent) : QLabel(text, parent) {}

private:
    void set_highlighted(bool highlighted)
    {
        // self-tinkered highlighting
        setBackgroundRole(highlighted ? QPalette::Highlight : QPalette::Window);
        setAutoFillBackground(highlighted);
    }

protected:
    void enterEvent(QEvent*) override {
        // `enterEvent` is not even called on mouse hover until other sub-menu closes.
        qDebug() << "Enter MyActionWidget(" << text() << ")";
        set_highlighted(true);
    }
    void leaveEvent(QEvent*) override {
        qDebug() << "Leave MyActionWidget(" << text() << ")";
        set_highlighted(false);
    }
};

class MyAction : public QWidgetAction
{
public:
    MyAction(const QString& text, QObject* parent) : QWidgetAction(parent), m_text(text) {}
#ifdef REPRODUCE_FIRST_ANIMATION
    QWidget* createWidget(QWidget* parent) override { return QWidgetAction::createWidget(parent); }  // works as expected; but does not produce a MyActionWidget
#else
    QWidget* createWidget(QWidget* parent) override { return new MyActionWidget(m_text, parent); }
#endif
private:
    const QString m_text;
};


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow w;

    QMenu* menu = new QMenu("The Menu");
    menu->addAction("Standard QAction 1");
    menu->addAction("Standard QAction 2");

    QMenu* sub_menu = new QMenu("sub menu", menu);
    sub_menu->addAction("sub action 1");
    sub_menu->addAction("sub action 2");
    menu->addMenu(sub_menu);
    menu->addAction(new MyAction("My Action 1", menu));
    menu->addAction(new MyAction("My Action 2", menu));
    menu->addAction("Standard QAction 3");
    menu->addAction("Standard QAction 4");

    QMenuBar* menu_bar = new QMenuBar();
    menu_bar->addMenu(menu);
    w.setMenuBar(menu_bar);
    w.setCentralWidget(new QTextEdit());
    w.setWindowTitle("Menu Example");

    w.show();

    return a.exec();
}

menu_example.pro

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = menu
TEMPLATE = app
SOURCES += main.cpp