Strange graphics scene bug in Qt 5.7?

[schollii]

We are hoping to upgrade to PyQt 5.7/Qt 5.7 but have this one nasty issue left to fix, and it looks like it could be at the Qt level. I have created a minimal Qt app using PyQt to reproduce the problem (see my post on stackoverflow). Here is what the view looks like:

This shows a graphics view containing two graphics items, each one identical (although that doesn't matter), and each one contains one proxy widget item (which can contain anything, here it contains a label) and one ellipse item (again could be anything like rectangle, poly etc). The image clearly shows how the ellipse "bleeds" into the surrounding area!

I converted the Python code from this example to C++, but I don't have a C++ dev env readily available and have not built Qt apps before (I have only used Qt from PyQt, albeit in rather large apps). So there may be a couple little compilation errors, can anyone try to make this build, then report if they can see the bug (and post the fixed code)? Here is the code:

#include <QApplication>
#include <QtCore>
#include <QtWidgets>
#include <QtGui>


class MyGraphicsItem: public QGraphicsObject
{
public:
    MyGraphicsItem::MyGraphicsItem()
    {
        # next line could be any type of graphics item:
        QGraphicsEllipseItem* rect_item = QGraphicsEllipseItem(0, 0, 100, 100, this);
        # effect easier to see if paint black:
        rect_item->setBrush(QBrush(Qt.SolidPattern));

        QGraphicsProxyWidget* label_item = QGraphicsProxyWidget(this);
        # *** Next line must be there for effect to be visible, but could be any other type of widget
        label_item->setWidget(new QLabel('a'*30));
    }

    QRectF boundingRect() const Q_DECL_OVERRIDE
    {
        return childrenBoundingRect();
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0) Q_DECL_OVERRIDE
    {
    }

private:
    QBrush* __default_brush;
    QGraphicsRectItem* __widget_rect;
    QGraphicsProxyWidget* __part_item;
    QPainterPath* __background_path;

};


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

    QWidget widget;
    QHBoxLayout* layout = QHBoxLayout();
    widget.setLayout(layout)

    QGraphicsView* view = new QGraphicsView();
    MyScene* scene = new QGraphicsScene();
    scene.addItem(MyGraphicsItem())  # *** effect only there if more than 1 item
    scene.addItem(MyGraphicsItem())
    view->setScene(scene);
    layout->addWidget(view);

    widget.setGeometry(100, 100, 50, 50)
    widget.show();
    return app.exec();
}

[SGaist]

Hi,

There's indeed something strange going on.

You should check the bug report system to see if it something know.

Here's a buildable version of your code:

#include <QApplication>
#include <QtCore>
#include <QtWidgets>
#include <QtGui>


class MyGraphicsItem: public QGraphicsObject
{
public:
    MyGraphicsItem()
    {
        // # next line could be any type of graphics item:
        QGraphicsEllipseItem* rect_item = new QGraphicsEllipseItem(0, 0, 100, 100, this);
        // # effect easier to see if paint black:
        rect_item->setBrush(QBrush(Qt::SolidPattern));

        QGraphicsProxyWidget* label_item = new QGraphicsProxyWidget(this);
        //# *** Next line must be there for effect to be visible, but could be any other type of widget
        label_item->setWidget(new QLabel("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"));
    }

    QRectF boundingRect() const Q_DECL_OVERRIDE
    {
        return childrenBoundingRect();
    }

private:
    QBrush* __default_brush;
    QGraphicsRectItem* __widget_rect;
    QGraphicsProxyWidget* __part_item;
    QPainterPath* __background_path;

};


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

    QWidget widget;
    QHBoxLayout* layout = new  QHBoxLayout();
    widget.setLayout(layout);

    QGraphicsView* view = new QGraphicsView();
    QGraphicsScene* scene = new QGraphicsScene();
    scene->addItem(new MyGraphicsItem());//  # *** effect only there if more than 1 item
    scene->addItem(new MyGraphicsItem());
    view->setScene(scene);
    layout->addWidget(view);

    widget.setGeometry(100, 100, 50, 50);
    widget.show();
    return app.exec();
}

[schollii]

Awesome, thanks so much for doing this. Indeed I found a bug report that is surely the same problem, and points to a workaround that seems to work in PyQt 5.7 example app and our full PyQt app, can you try this patch to confirm on the C++ side:

...
label_item->setWidget(new QLabel("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"));
label_item->setOpacity(0.99999);  // ADD THIS LINE

Not a fix but an acceptable workaround, hopefully the Qt bug gets addressed eventually.