在pyq中更改大小时，图形上的黑色边框显示为NVAS

Question

我正在尝试将matplotlib（版本3.0.0）图形嵌入到pyqt5gui（版本5.9.2）中。因此，我正在创建一个画布并将其添加到布局中。但是，当我开始更改大小属性（如canvas.setFixedSize(500,500)），然后通过拖动窗口的一角来更改gui的大小时，我会得到一个黑色的边框：。你知道吗

这种行为可以通过添加fig.patch.set_facecolor('#ffffff')来修复。然而，我想知道这种行为是否是预期的，或者是否有另一种方法来防止黑边界出现？你知道吗

import sys
from PyQt5.QtWidgets import (QWidget, QGridLayout,QPushButton, QApplication, QSizePolicy)

from matplotlib.backends.qt_compat import QtCore, QtWidgets, is_pyqt5
if is_pyqt5():
    from matplotlib.backends.backend_qt5agg import (
        FigureCanvas, NavigationToolbar2QT as NavigationToolbar)
else:
    from matplotlib.backends.backend_qt4agg import (
        FigureCanvas, NavigationToolbar2QT as NavigationToolbar)
from matplotlib.figure import Figure
import matplotlib.pyplot as plt

class ApplicationWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        self._main = QtWidgets.QWidget()
        self.setCentralWidget(self._main)
        layout = QGridLayout(self._main)        

        fig, ax = plt.subplots(1,1)
        canvas = FigureCanvas(fig)
        # enabling one of following options leads to black frame
        #canvas.setSizePolicy(QSizePolicy.Fixed, QSizePolicy.Fixed) 
        #canvas.setFixedSize(500,500)

        #which can be fixed by
        #fig.patch.set_facecolor('#ffffff')

        layout.addWidget(canvas)

if __name__ == "__main__":
    # Check whether there is already a running QApplication (e.g., if running
    # from an IDE).
    qapp = QtWidgets.QApplication.instance()
    if not qapp:
        qapp = QtWidgets.QApplication(sys.argv)

    app = ApplicationWindow()
    app.show()
    qapp.exec_()