Why a custom QWidget doesn't show in a QGridLayout?

Question

I'm trying to build a custom toolbar for my application. But the toolbar doesn't show when I put it in the QGridLayout of the application.

This is a test code that regenerates the issue.

from sys import argv
from sys import exit as ex
from PyQt6.QtCore import Qt
from PyQt6.QtWidgets import QApplication, QGridLayout, QTextEdit, QWidget, QSizePolicy


class Toolbar(QWidget):
    def __init__(
        self,
        parent: QWidget | None = None,
        flags: Qt.WindowType = Qt.WindowType.Widget,
    ) -> None:
        super().__init__(parent, flags)
        self.setMinimumHeight(50)


class MainWindow(QWidget):
    def __init__(
        self, parent: QWidget | None = None, flags: Qt.WindowType = Qt.WindowType.Window
    ) -> None:
        super().__init__(parent, flags)
        self.__setup_ui()

    def __setup_ui(self) -> None:
        self.grid_layout = QGridLayout()
        toolbar = Toolbar()
        self.grid_layout.addWidget(toolbar, 0, 0, 1, 4)
        text_edit = QTextEdit()
        self.grid_layout.addWidget(text_edit, 1, 0, 2, 1)
        text_edit = QTextEdit()
        self.grid_layout.addWidget(text_edit, 1, 1, 1, 1)
        text_edit = QTextEdit()
        self.grid_layout.addWidget(text_edit, 1, 2, 2, 1)
        text_edit = QTextEdit()
        self.grid_layout.addWidget(text_edit, 1, 3, 2, 1)
        text_edit = QTextEdit()
        self.grid_layout.addWidget(text_edit, 2, 1, 1, 1)
        self.setLayout(self.grid_layout)


if __name__ == "__main__":
    application = QApplication(argv)
    main_window = MainWindow()
    main_window.show()
    ex(application.exec())

This is what it looks like when we run it.

When I put a QTextEdit instead the toolbar.

toolbar = QTextEdit()

I get this.

I saw this.

https://forum.qt.io/topic/97929/custom-qwidget-doesn-t-show-up-in-qgridlayout-only-in-a-new-window/7

So I tried this.

self.grid_layout.addWidget(toolbar.setVisible(True), 0, 0, 1, 4)

I found this.

https://doc.qt.io/qtforpython-6/overviews/layout.html

That says:

Custom Widgets in Layouts When you make your own widget class, you should also communicate its layout properties. If the widget uses one of Qt’s layouts, this is already taken care of. If the widget does not have any child widgets, or uses a manual layout, you can change the behavior of the widget using any or all of the following mechanisms:

• Reimplement sizeHint() to return the preferred size of the widget.
• Reimplement minimumSizeHint() to return the smallest size the widget can have.
• Call setSizePolicy() to specify the space requirements of the widget.

So I tried this.

class Toolbar(QWidget):
    def __init__(
        self,
        parent: QWidget | None = None,
        flags: Qt.WindowType = Qt.WindowType.Widget,
    ) -> None:
        ...
        self.setSizePolicy(
            QSizePolicy(QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Minimum)
        )

But I got same result.

Could you help me figure out why this custom QWidget does not show, please.