Suppose you designed an app where the child window exits. Your user may close the child window by clicking the close button. You may, however, disable the window as read-only with setReadOnly to true as setReadOnly(true). You may also make it invisible with CSS display:none, but doing so means you have just created a security hole with the CSS key. This is because a malicious user can access the button and close the window. The best pratice is to set the window to read-only, which also prevents closing on the server side.

Here is an example of the use of a child window in an application using a custom component with open and close button:

class WindowHole extends CustomComponent implements Window.CloseListener {
    Window mainwindow;
    Window mywindow;
    Button openbutton;
    Button closebutton;
    Label explanation;
    public WindowHole(String label, Window main) {
        mainwindow = main;
        final VerticalLayout layout = new VerticalLayout();
        openbutton = new Button("Open Window", this,"openButtonClick");
        explanation = new Label("Explanation");
        layout.addComponent(openbutton);
        layout.addComponent(explanation);
        setCompositionRoot(layout);
    }
    public void openButtonClick(Button.ClickEvent event) {
        mywindow = new Window("My Dialog");
        mywindow.setPositionX(200);
        mywindow.setPositionY(100);
        mainwindow.addWindow(mywindow);
        mywindow.addListener(this);
        mywindow.addComponent(new Label("A text label in the window."));
        closebutton = new Button("Close", this, "closeButtonClick");
        mywindow.addComponent(closebutton);
        openbutton.setEnabled(false);
        explanation.setValue("Window opened");
    }
    public void closeButtonClick(Button.ClickEvent event) {
        mainwindow.removeWindow(mywindow);
        openbutton.setEnabled(true);
        explanation.setValue("Closed with button");
    }
    public void windowClose(CloseEvent e) {
        openbutton.setEnabled(true);
        explanation.setValue("Closed with window controls");
    }
}

This example shows how we can inherit the CustomComponent class which consists of a button for the window opening and a label to show the attributes of the window.

Now you can see the window is open but the button is disabled. When the window is closed the button is enabled.

I hope this helps!