[REG 6.3.2 -> 6.4.0] Mac: QWindow blanks when resized by a custom window manager when it contains a QOpenGLWidget

When using a window manager such as Rectangle, the window will blank to grey when resized by the manager if it contains a QOpenGLWidget. This doesn't happen if it only contains regular widget.

This can be demonstrated with this simple program:

from PySide6.QtGui import QPaintEvent, QPainter, QColor
from PySide6.QtOpenGLWidgets import QOpenGLWidget
from PySide6.QtWidgets import QApplication, QMainWindow, QPushButton


class MyWidget(QOpenGLWidget):
 def paintEvent(self , event: QPaintEvent):
 painter = QPainter(self)
 painter.fillRect(0, 0, 50, 50, QColor.fromRgb(255, 255, 128))
 painter.end()


class MainWindow(QMainWindow):
 def __init__(self):
 super().__init__()

 self.setCentralWidget(MyWidget())


app = QApplication(sys.argv)

window = MainWindow()
window.show()

app.exec()

(sorry, I'm unable to provide a C++ version of the same)

Here is a recording of the behaviour:

Note that for a resizeGL()/paintGL() implementation, resizeGL() is called when the window manager resizes the window, but paintGL() is not.

If MyWidget() is replaced by some other widget (e.g. a button), the behaviour is as expected.