How to disable the delivery of mouse events to the widget but not its children in Qt?

At last I found a solution :)

QWidget::setMask ( const QRegion & region )

https://doc.qt.io/qt-5/qwidget.html#setMask-1

http://qt-project.org/doc/qt-4.8/qwidget.html#setMask

I found the solution here: http://www.qtcentre.org/archive/index.php/t-3033.html

QRegion reg(frameGeometry());
reg -= QRegion(geometry());
reg += childrenRegion();
setMask(reg);

Now children of the front widget and the widgets behind the front widget respond to the mouse events as required!

Remember, you would need to call these lines again whenever the front widget is re-sized to recalculate the geometry for the mask!

void someWidget::resizeEvent(QResizeEvent *e){
  QWidget::resizeEvent(e);
  QRegion reg(frameGeometry());
  reg-=QRegion(geometry()); 
  reg+=childrenRegion();
  setMask(reg);
}

The solution of the OP is awesome and very elegant. Just for completeness, another option would be to ignore mouse events when they reach the container widget. It can be done either by sub-classing or through a eventFilter.

This solution can be helpful if the widget hides/shows many children widgets dynamically and computing the mask becomes difficult.

Note: In the case you want to track mouse-move events in background widgets, you'd have to setMouseTracking(true) to receive (and then ignore) the QEvent::MouseMove when no button is pressed.

Example by sub-classing

ContainerWidget::ContainerWidget(...) {
  setMouseTracking(true);
}

void ContainerWidget::mouseMoveEvent(QMouseEvent* e) {
  e->ignore();
}
void ContainerWidget::mousePressEvent(QMouseEvent* e) {
  e->ignore();
}

Example using the event filter

// Assume the container widget is configured in the constructor
MainWindow::MainWindow(...) {
  // ...
  containerWidget->installEventFilter(this);
  containerWidget->setMouseTracking(true);
  // ...
}

bool MainWindow::eventFilter(QObject* o, QEvent* e) {
  if (o == containerWidget &&
     (e->type() == QEvent::MouseMove || e->type() == QEvent::MouseButtonPress)) {
    e->ignore();
    return false;
  }
  return QMainWindow::eventFilter(o, e);
}