QWidget::mapToGlobal() not giving right result

JonB

Summary: In a nutshell, is QWidget::mapToGlobal() supposed to give correct result if the widget is on a [box] layout?

I am using maptoGlobal() to display a popup window off a QPushButton. I want the popup window to appear right where the button is. So I am using just what you would expect:

QWidget _popup;    // no parent
_popup.setWindowFlags(Qt::Popup | Qt::FramelessWindowHint);

// this is QPushButton, with parentage, on layout
connect(this, &QPushButton::clicked, this, &UIUtils::PushButtonWithPopup::doPopup);

/*slot*/ void UIUtils::PushButtonWithPopup::doPopup()
{
    QPoint pos = this->mapToGlobal(this->pos());
    _popup.show();
    _popup.move(pos.x(), pos.y());
}

What I am finding is: when the button is at the lefthand side of window --- i.e. pos.x() ==~ 0 --- the global position is good. But as it moves right --- pos.x() > 100 etc. --- mapToGlobal(pos()) is returning an x() which is getting larger & larger, so the popup appears further & further to the right of the button. The y() however remains correct.

Now, the button is on a QHBoxlayout. I started to read an old post
https://www.qtcentre.org/threads/40574-mapToGlobal-mapFromGlobal-isn-t-working-properly
which said at one point

So it is managed by a layout which might explain why your own geometry management fails. If you wish to do your own moving/resizing, you can't use layouts at the same time.

Is that what is going on?

Please don't ask me to repro a full example for now, it will take too long. I would just like to quickly know: if I place a widget on a layout, does that mean I won't be able to use mapToGlobal(pos()) to get the correct global screen coordinate to position a popup at? If so, how can I map from a widget on a layout to its correct screen position? Thanks.

Chris Kawa

@JonB said in QWidget::mapToGlobal() not giving right result:

QPoint pos = this->mapToGlobal(this->pos());

That's wrong. pos() is a position in the parent, and this->mapToGlobal maps it relatively to this, not the parent, so simply said - you're mixing different coordinate systems - local and parent. Additionally, in case of a top level widget that has no parent, pos() is already a global position.

If you want the global position of the button you either map its position in parent relative to that parent:

QPoint global_pos = btn->parentWidget()->mapToGlobal(btn->pos());

or, simpler, and IMO more appropriate here, just map the upper left corner of your button relative to it:

QPoint global_pos = btn->mapToGlobal(QPoint(0,0));

J.Hilk

@JonB I can reproduce it:

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

    QWidget w;
    QLabel l;
    l.setStyleSheet("background-color:red;");
    w.resize(200,50);
    QHBoxLayout *hLay = new QHBoxLayout(&w);

    for(int i(0); i <9; i++) {
        QPushButton *btn = new QPushButton(QString("Btn%1").arg(i));
        QObject::connect(btn, &QPushButton::clicked, &l, [=, &l]()->void{
            QPoint p = btn->mapToGlobal(btn->pos());
            l.resize(btn->size());
            l.move(p);
        });
        hLay->addWidget(btn);
    }

    w.show();
    l.show();

    return app.exec();
}

Chris Kawa

@JonB said in QWidget::mapToGlobal() not giving right result:

QPoint pos = this->mapToGlobal(this->pos());

That's wrong. pos() is a position in the parent, and this->mapToGlobal maps it relatively to this, not the parent, so simply said - you're mixing different coordinate systems - local and parent. Additionally, in case of a top level widget that has no parent, pos() is already a global position.

If you want the global position of the button you either map its position in parent relative to that parent:

QPoint global_pos = btn->parentWidget()->mapToGlobal(btn->pos());

or, simpler, and IMO more appropriate here, just map the upper left corner of your button relative to it:

QPoint global_pos = btn->mapToGlobal(QPoint(0,0));

J.Hilk

@Chris-Kawa +1 that explains so much of my past experience 🤦‍♂️

with that in mind, this works now as well:

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

    QWidget w;
    QLabel l;
    l.setStyleSheet("background-color:red;");
    w.resize(500,50);
    QHBoxLayout *hLay = new QHBoxLayout(&w);

    for(int i(0); i <9; i++) {
        QPushButton *btn = new QPushButton(&w);
        btn->setText(QString("Btn%1").arg(i));
        QObject::connect(btn, &QPushButton::clicked, &l, [=, &l, &w]()->void{
            QPoint p = w.mapToGlobal(btn->pos());
            l.resize(btn->size());
            l.move(p);
        });
        hLay->addWidget(btn);
    }

    w.show();
    l.show();

    return app.exec();
}

JonB

Thank you both for prompt responses!

@Chris-Kawa

Additionally, in case of a top level widget that has no parent, pos() is already a global position.

I'm a little lost here. I am trying to get the button's coordinates, the button does have parentage. It is the popup which will not have parentage, and so need global coordinates...?

Meanwhile, I am about to try out your solutions and will report back....

UPDATE
Still not sure what you meant by

QPoint global_pos = btn->mapToGlobal(QPoint(0,0));

~~That's what I had originally (this == btn); I think you meant that if it had no parent?~~ [EDIT No, see below]

Anyway, altered to

QPoint pos = this->parentWidget()->mapToGlobal(this->pos());

and lo & behold the global position is correct, whether the button is at left or right of its parent/layout.

You are an genius, thank you! :)

EDIT
OIC, btn->mapToGlobal(QPoint(0,0)), that's QPoint(0, 0) not btn->pos(). Got it! So now I have:

QPoint pos = this->mapToGlobal(QPoint(0, 0));

I understand what's going on!!