How to pass click event to sibling underneath in Qt?

Question

I have two overlapping widgets in a window child A and child B. Child A is above B and gets mouse events but sometimes the click should pass through and eventually hit child B.

The ideal solution would have been to use event->ignore() but this passes the event to the parent widget, not siblings.

The "pass all" solution setAttribute(Qt::WA_TransparentForMouseEvents); does not work either because child A needs to capture some events.

How do I tell Qt "I do not want to handle this event, act like I am not there" ?

Answer 1

The easier solution (if applicable) is to set WA_TransparentForMouseEvents depending on your click-through condition.

For example, if you want to click through certain regions of Child-A, you can use its mouseMoveEvent() to check whether WA_TransparentForMouseEvents needs to be set or unset. Then click events pass through automatically.

If you cannot determine whether a click event should be accepted before it has actually been triggered, you can do something like this:

void ChildA::mousePressEvent(QMouseEvent* event) {
    const bool accepted = ...; //insert custom logic here
    if (accepted) {
        ... //handle event here
    } else {
        //not accepting event -> resend a copy of this event...
        QMouseEvent* eventCopy = new QMouseEvent(*event);
        QApplication::instance()->postEvent(eventCopy);
        //...but this time, ignore it
        setAttribute(Qt::WA_TransparentForMouseEvents, true);
        QTimer::singleSlot(1, this, SLOT(resetClickTransparency()));
        //don't propagate original event any further
        event->accept();
    }
}

void ChildA::resetClickTransparency() { //slot
    setAttribute(Qt::WA_TransparentForMouseEvents, false);
}

Disclaimer: All of this written down by heart after a year of not doing Qt, so please correct me on name or type errors.