Why QGraphicsProxyWidget does not resize properly after dropping into QGraphicsView?

Question

I have a QListWidget and a QGraphicsView both subclassed to overwrite some of their members. I prepared a minimal verifiable example showing the proble

Answer 1

Cause

You create proxyControl giving wgt->width() as a width:

QGraphicsRectItem *proxyControl = addRect(initPos.x(), initPos.y(), wgt->width(), 20, QPen(Qt::black), QBrush(Qt::darkGreen));

However, at that time wgt does not have its final geometry.

Solution

Setup proxyControl after you are done with wgt.

Example

Here is an example I have written for you to demonstrate how the proposed solution could be implemented. Change your dropEvent like this:

void Scene::dropEvent(QGraphicsSceneDragDropEvent *event)
{
    QByteArray encoded =
            event->mimeData()->data("application/x-qabstractitemmodeldatalist");
    QDataStream stream(&encoded, QIODevice::ReadOnly);

    QStringList rosTables;
    QString newString;

    while (!stream.atEnd()) {
        int row, col;
        QMap<int, QVariant> roleDataMap;
        stream >> row >> col >> roleDataMap;
        rosTables << roleDataMap[Qt::DisplayRole].toString();
    }

    for (const QString &tableType : rosTables) {
        if (tableType == "Images") {
            QPoint initPos(0, 0);
            auto *wgt = new CustomTableWidget;
            auto *proxyControl = addRect(0, 0, 0, 0, QPen(Qt::black),
                                         QBrush(Qt::darkGreen));
            auto *sizeGrip = new QSizeGrip(wgt);
            auto *layout = new QHBoxLayout(wgt);

            layout->setContentsMargins(0, 0, 0, 0);
            layout->addWidget(sizeGrip, 0, Qt::AlignRight | Qt::AlignBottom);

            connect(wgt, &CustomTableWidget::sizeChanged, [wgt, proxyControl](){
                proxyControl->setRect(wgt->geometry().adjusted(-10, -10, 10, 10));
            });

            wgt->setColumnCount(2);
            wgt->setRowCount(2);

            for (int ridx = 0; ridx < wgt->rowCount(); ridx++) {
                for (int cidx = 0; cidx < wgt->columnCount(); cidx++) {
                    auto *item = new QTableWidgetItem();

                    item->setText(QString("%1").arg(ridx));
                    wgt->setItem(ridx,cidx,item);
                }
            }

            auto *const proxy = addWidget(wgt);


            proxy->setPos(initPos.x(), initPos.y()
                          + proxyControl->rect().height());
            proxy->setParentItem(proxyControl);

            proxyControl->setPos(initPos.x(), initPos.y());
            proxyControl->setFlag(QGraphicsItem::ItemIsMovable, true);
            proxyControl->setFlag(QGraphicsItem::ItemIsSelectable, true);
            proxyControl->setRect(wgt->geometry().adjusted(-10, -10, 10, 10));
        }
    }
}

Note: Use QGraphicsSceneDragDropEvent::scenePos() to properly position the dropped item.

Result

The proposed change gives you the desired result right away: