QWidget do not trigger QEvent::MouseMove when entered with Pressed button

Question

In Qt with C++, I created a window with a small QWidget inside.

The small QWidget show a message every time QEvent::Enter, QEvent::Leave or QEvent::MouseMove is triggered.

When any mouse button is pressed (and holded) outside of the small QWidget, and the mouse is moved on the top of this small QWidget (While holding), QEvent::MouseMove is not triggered for this small QWidget. Additionally, QEvent::Enter is postponed to after the mouse button is released.

In the reverse situation: when the mouse is pressed on the small QWidget (and holded), and then the mouse is moved outside, the QEvent::Leave is postponed to after the mouse button is released.

IS there any solution to retrieve QEvent::MouseMove all the time, even when the mouse button is holded?

Additional data: Yes, setMouseTracking(true) is set.

Testing example:

Widget:

#ifndef MYWIDGET_HPP
#define MYWIDGET_HPP

#include <QWidget>
#include <QStyleOption>
#include <QPainter>
#include <QEvent>
#include <QDebug>

class MyWidget: public QWidget
{
    Q_OBJECT
public:
    MyWidget( QWidget* parent=nullptr ): QWidget(parent)
    {
        setMouseTracking(true);
    }
protected:

    // Paint for styling
    void paintEvent(QPaintEvent *)
    {
        // Needed to allow stylesheet.
        QStyleOption opt;
        opt.init(this);
        QPainter p(this);
        style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);
    }

    // Show Enter and Leave event for debugging purpose
    bool event( QEvent *e)
    {
        static int counting=0;
        if (e->type() ==QEvent::Enter)
        {
            qDebug() << counting++ << " Enter: " << this->objectName();
        }
        if (e->type() ==QEvent::Leave)
        {
            qDebug() << counting++ << " Leave: " << this->objectName();
        }

        if (e->type() ==QEvent::MouseMove)
        {
            qDebug() << counting++ << " Move: " << this->objectName();
        }
        return QWidget::event(e);
    }

};

#endif // MYWIDGET_HPP

Main

#include <QApplication>

#include <QDebug>
#include <QWidget>
#include <QTimer>

#include "Testing.hpp"


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

    // Create a main window
    QWidget main;
    main.setWindowTitle("Cursor blocked for 5s - wait and see");
    main.resize(500, 200);
    main.move(200, 200);

    // Create a MyWidget
    MyWidget sub(&main);
    sub.setObjectName("sub");
    sub.resize(50, 50);
    sub.move(50, 50);

    // Style the button with a hover
    main.setStyleSheet
    (
        "QWidget#sub{background-color: rgba(0,0,128,0.5);}"
        "QWidget#sub:hover{background-color: rgba(128,0,0,0.5);}"
    );

    // Show the window
    main.show();

    return a.exec();

}

Project

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

SOURCES +=\
    main.cpp

HEADERS  +=\
    Testing.hpp

RESOURCES +=\

CONFIG += c++11 -Wall

TARGET = Testing
TEMPLATE = app

Answer 1

It is standard behavior. When you press mouse button, widget begin to grab it (make a call of QWidget::grabMouse). I think that you should redesign your behavior, or explain some real use-cases, when you need to track mouse globally.

If you really need to track mouse, you may use event filters.

Pseudo-code (without checks):

QWidget *otherWidget = /*...*/;
QWidget *myWidget  = /*...*/;
otherWidget->installEventFilter( myWidget );
// you need to install filter on each widget,
// that you want to track.
// Care with performance

MyWidget : QWidget
{
  void handleMouseMove( QPoint pos ) { /*...you code...*/ }

  void mouseMove( QMouseEvent *e ) override;
  {
    handleMouseMove( e->pos() );
    QWidget::mouseMove( e );
  }

  bool eventFilter( QObject *obj, QEvent *e )
  {
    auto srcWidget = qobject_cast< QWidget * >( obj );
    switch ( e->type() )
    {
    case QEvent::MouseMove:
      {
        auto me = static_cast< QMouseEvent * >( e );
        auto globalPos = srcWidget->mapToGlobal( me->pos() );
        auto localPos = this->mapFromGlobal( globalPos ); // Possible, you need to invalidate that poing belongs to widget
        handleMouseMove( localPos );
      }
      break;
    };
    return QWidget::eventFilter( obj, e );
  }
};