QApplication::activeModalWidget()
A modal widget is a special top-level widget which is a subclass of QDialog that specifies the modal parameter of the constructor as true. A modal widget must be closed before the user can continue with other parts of the program.

QApplication::activePopupWidget()
A popup widget is a special top-level widget that sets the Qt::WType_Popup widget flag, e.g. the QMenu widget. When the application opens a popup widget, all events are sent to the popup. Normal widgets and modal widgets cannot be accessed before the popup widget is closed.

Questions

Custom popup widget
How would I go about making a overlay widget

Code

void QCompleter::setPopup(QAbstractItemView *popup)
{
    Q_D(QCompleter);
    Q_ASSERT(popup != nullptr);
    if (d->popup) {
        QObject::disconnect(d->popup->selectionModel(), nullptr, this, nullptr);
        QObject::disconnect(d->popup, nullptr, this, nullptr);
    }
    if (d->popup != popup)
        delete d->popup;
    if (popup->model() != d->proxy)
        popup->setModel(d->proxy);
     popup->hide();

    Qt::FocusPolicy origPolicy = Qt::NoFocus;
    if (d->widget)
        origPolicy = d->widget->focusPolicy();

    // Mark the widget window as a popup, so that if the last non-popup window is closed by the
    // user, the application should not be prevented from exiting. It needs to be set explicitly via
    // setWindowFlag(), because passing the flag via setParent(parent, windowFlags) does not call
    // QWidgetPrivate::adjustQuitOnCloseAttribute(), and causes an application not to exit if the
    // popup ends up being the last window.
    popup->setParent(nullptr);
    popup->setWindowFlag(Qt::Popup);
    popup->setFocusPolicy(Qt::NoFocus);
    if (d->widget)
        d->widget->setFocusPolicy(origPolicy);

    popup->setFocusProxy(d->widget);
    popup->installEventFilter(this);
    popup->setItemDelegate(new QCompleterItemDelegate(popup));
#if QT_CONFIG(listview)
    if (QListView *listView = qobject_cast<QListView *>(popup)) {
        listView->setModelColumn(d->column);
    }
#endif

    QObject::connect(popup, SIGNAL(clicked(QModelIndex)),
                     this, SLOT(_q_complete(QModelIndex)));
    QObject::connect(this, SIGNAL(activated(QModelIndex)),
                     popup, SLOT(hide()));

    QObject::connect(popup->selectionModel(), SIGNAL(selectionChanged(QItemSelection,QItemSelection)),
                     this, SLOT(_q_completionSelected(QItemSelection)));
    d->popup = popup;
}

void QCompleterPrivate::showPopup(const QRect& rect)
{
    const QRect screen = QWidgetPrivate::availableScreenGeometry(widget);
    Qt::LayoutDirection dir = widget->layoutDirection();
    QPoint pos;
    int rh, w;
    int h = (popup->sizeHintForRow(0) * qMin(maxVisibleItems, popup->model()->rowCount()) + 3) + 3;
    QScrollBar *hsb = popup->horizontalScrollBar();
    if (hsb && hsb->isVisible())
        h += popup->horizontalScrollBar()->sizeHint().height();

    if (rect.isValid()) {
        rh = rect.height();
        w = rect.width();
        pos = widget->mapToGlobal(dir == Qt::RightToLeft ? rect.bottomRight() : rect.bottomLeft());
    } else {
        rh = widget->height();
        pos = widget->mapToGlobal(QPoint(0, widget->height() - 2));
        w = widget->width();
    }

    if (w > screen.width())
        w = screen.width();
    if ((pos.x() + w) > (screen.x() + screen.width()))
        pos.setX(screen.x() + screen.width() - w);
    if (pos.x() < screen.x())
        pos.setX(screen.x());

    int top = pos.y() - rh - screen.top() + 2;
    int bottom = screen.bottom() - pos.y();
    h = qMax(h, popup->minimumHeight());
    if (h > bottom) {
        h = qMin(qMax(top, bottom), h);

        if (top > bottom)
            pos.setY(pos.y() - h - rh + 2);
    }

    popup->setGeometry(pos.x(), pos.y(), w, h);

    if (!popup->isVisible())
        popup->show();
}

Transparent/Alpha Background

QDialog with transparent background color
How not to lose the alpha channel

QML

Documentation

Qt 5.15 – Qt Quick Controls – Popup Controls

Questions

Popup item loses opaquness
how to set popup background color to transparent
Change popup background to transparent