为什么combobox的show popup功能在qt中不起作用？

from PyQt5.QtWidgets import QApplication, QComboBox, QWidget, QVBoxLayout
from PyQt5.QtCore import QEvent

class MyWidget(QWidget):
    def __init__(self):
        super().__init__()
        layout = QVBoxLayout(self)
        self.combo_box = QComboBox()
        self.combo_box.addItems(["Option 1", "Option 2", "Option 3"])
        self.combo_box.setParent(self)
        layout.addWidget(self.combo_box)
        self.combo_box.installEventFilter(self)

    def eventFilter(self, obj, event):
        if event.type() == QEvent.MouseButtonPress:
            self.combo_box.showPopup()
            return True
        return super().eventFilter(obj, event)

app = QApplication([])
window = MyWidget()
window.show()
app.exec_()