[PyQt] Resize flicking on QMainWindow centralWidget in python but not C++

[Patrick Stinson]

Turns out simply adding a QQuickWidget as a child to QMainWindow::centralWidget causes this problem. I imagine this is because of the OpenGL buffer in QtQuick:

    app = QApplication(sys.argv)
    mw = QMainWindow()
    w = QQuickWidget()
    mw.setCentralWidget(w)
    mw.show()
    app.exec()

Does anyone know of a workaround for this? It makes the app look so ugly!

> On Oct 7, 2019, at 2:10 PM, Eric Pettersen <pett at cgl.ucsf.edu> wrote:
> 
> We had this behavior in our own app, but it kind of went away on its own.  We came to suspect that it had to with the setting of MACOSX_DEPLOYMENT_TARGET.  I don’t know if you set that variable in your compile, but if you do you might try changing it (or not setting it) and see what happens.
> 
> —Eric
> 
>> On Oct 6, 2019, at 1:38 PM, Patrick Stinson <patrickkidd at gmail.com <mailto:patrickkidd at gmail.com>> wrote:
>> 
>> I wonder if maybe the use of QQuickWidget child widgets enables some kind of OpenGL buffer in the QMainWindoe?
>> 
>>> On Oct 6, 2019, at 12:25 AM, Patrick Stinson <patrickkidd at gmail.com <mailto:patrickkidd at gmail.com>> wrote:
>>> 
>>> I put a qDebug() line in QMainWindow() and QOpenGLWidget(), rebuilt/installed Qt, and ran my app. I saw the line for the QMainWindow() but not QOpenGLWidget(), so that means QOpenGLWidget isn’t being run in my app. But I am still seeing the problem. Difficult to pinpoint as a reproducible report.
>>> 
>>>> On Oct 5, 2019, at 10:25 PM, Patrick Stinson <patrickkidd at gmail.com <mailto:patrickkidd at gmail.com>> wrote:
>>>> 
>>>> So it does, and changing that makes it work like the C++ example.
>>>> 
>>>> My own app is still showing the problem even without using QOpenGLWidget. I wonder if there is some setting somewhere that implicitly uses QOpenGLWidget….
>>>> 
>>>>> On Oct 5, 2019, at 5:36 PM, Eric Pettersen <pett at cgl.ucsf.edu <mailto:pett at cgl.ucsf.edu>> wrote:
>>>>> 
>>>>> Your Python class inherits from QOpenGLWidget.  Your C++ class inherits from QWidget.
>>>>> 
>>>>>> On Oct 5, 2019, at 2:10 PM, Patrick Stinson <patrickkidd at gmail.com <mailto:patrickkidd at gmail.com>> wrote:
>>>>>> 
>>>>>> Hello,
>>>>>> 
>>>>>> On MacOS, a QMainWindow with a simple painted centralWidget flickers when resizing quickly in python but not in C++. This only occurs with QMainWindow It is particularly pronounced more complex widgets, as shown in the following video:
>>>>>> 
>>>>>> https://youtu.be/0Qjy2wm6m9M <https://youtu.be/0Qjy2wm6m9M>
>>>>>> 
>>>>>> Here is the python example that shows the bug. Just run, rapidly resize the height of the widget , and observe a black flickering on the top edge of the widget as the height changes.
>>>>>> 
>>>>>> class Widget(QOpenGLWidget):
>>>>>>     def paintEvent(self, e):
>>>>>>         p = QPainter(self)
>>>>>>         p.setBrush(Qt.red)
>>>>>>         p.drawRect(self.rect())
>>>>>> 
>>>>>> app = QApplication(sys.argv)
>>>>>> mw = QMainWindow()
>>>>>> w = Widget()
>>>>>> mw.setCentralWidget(w)
>>>>>> mw.show()
>>>>>> w.show()
>>>>>> app.exec()
>>>>>> 
>>>>>> 
>>>>>> The following 1-to-1 C++ translation shows no such behavior:
>>>>>> 
>>>>>> 
>>>>>> #include <QApplication>
>>>>>> #include <QMainWindow>
>>>>>> #include <QWidget>
>>>>>> #include <QPainter>
>>>>>> 
>>>>>> class Widget : public QWidget {
>>>>>> public:
>>>>>>     void paintEvent(QPaintEvent *) {
>>>>>>         QPainter p(this);
>>>>>>         p.setBrush(Qt::red);
>>>>>>         p.drawRect(rect());
>>>>>>     }
>>>>>> };
>>>>>> 
>>>>>> int main(int argc, char *argv[])
>>>>>> {
>>>>>>     QApplication a(argc, argv);
>>>>>>     QMainWindow mw;
>>>>>>     QWidget *w = new Widget;
>>>>>>     mw.setCentralWidget(w);
>>>>>>     mw.show();
>>>>>>     return a.exec();
>>>>>> }
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
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>>>>> 
>>>> 
>>> 
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