Why does binding to Page.Title not work immediately?

I'm in the process of creating an installation wizard for my WPF application. To navigate between the installation steps I'm using a frame control with an array of pages that will be displayed. This is the main view and its view model:

<Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <Border Grid.Row="0"
                Background="{DynamicResource {x:Static ui:Brushes.Layer0BackgroundBrush}}"
                BorderBrush="{DynamicResource {x:Static ui:Brushes.Layer1BorderBrush}}"
                BorderThickness="0,0,0,1">
            
            <StackPanel Orientation="Horizontal">
                <Image Source="{DynamicResource AppIcon}"
                       Margin="10"
                       Height="48"
                       Width="48"/>

                <TextBlock Margin="5,10"
                           FontSize="24"
                           VerticalAlignment="Center"
                           Text="{Binding StepTitle}"/>
            </StackPanel>
        </Border>

        <Frame Grid.Row="1"
               Content="{Binding CurrentPage}"
               NavigationUIVisibility="Hidden"/>
    </Grid>

public class WizardMainViewModel : ObservableObject
{
    static readonly Page main = new WizardMainPage();
    static readonly Page selectEdition = new WizardSelectEditionPage();

    private static readonly Page[] wizardSteps = new Page[]
    {
        main,
        selectEdition
    };

    private int _stepIndex = 0;
    public int StepIndex
    {
        get => _stepIndex;
        set
        {
            SetProperty(ref _stepIndex, value);

            CurrentPage = wizardSteps[value];
            StepTitle = wizardSteps[value].Title;
        }
    }

    private Page _currentPage = wizardSteps[0];
    public Page CurrentPage
    {
        get => _currentPage;
        set => SetProperty(ref _currentPage, value);
    }

    private string _stepTitle = wizardSteps[0].Title;
    public string StepTitle
    {
        get => _stepTitle;
        set => SetProperty(ref _stepTitle, value);
    }
}

The TextBlock in the main view is supposed to display the title of the current page, but at first, it is empty. Every page has 'Back' and 'Next' buttons to switch pages:

private void Cancel(object sender, RoutedEventArgs e)
    {
        Window.GetWindow(this).Close();
    }

    private void Back(object sender, RoutedEventArgs e)
    {
        (Window.GetWindow(this).DataContext as WizardMainViewModel).StepIndex--;
    }

    private void Next(object sender, RoutedEventArgs e)
    {
        (Window.GetWindow(this).DataContext as WizardMainViewModel).StepIndex++;
    }

After you press 'Back', the correct title appears. Why is it not visible right away?

I tried this code with the MainViewModel instantiated in MainWindow.

That works for me.

My two pages have nothing in them.

My mainwindow has:

<Window.DataContext>
    <local:WizardMainViewModel/>
</Window.DataContext>
<Grid>
    <StackPanel>
        <TextBlock Margin="5,10"
                       FontSize="24"
                       VerticalAlignment="Center"
                       Text="{Binding StepTitle}"/>

    <Frame Grid.Row="1"
           Content="{Binding CurrentPage}"
           NavigationUIVisibility="Hidden"/>
    </StackPanel>
</Grid>
</Window>

You could try that approach.

If that doesn't work for you.

Instantiate WizardMainViewModel first, then set datacontext of the view to that instance.

I also tried

    private void Window_ContentRendered(object sender, EventArgs e)
    {
        var vm = new WizardMainViewModel();
        this.DataContext = vm;
    }

Which worked.

Your problem here is likely caused by the relative expense of instantiating pages compare to an int. Since I can't reproduce your issue, difficult to prove for certain.

Setting _stepindex to 0 will be very quick.

Building out pages will be far slower.

If the page isn't built when the view hits up that StepTitle getter then there's no title.

You should not be using pages because they have thread affinity and you can't build out this viewmodel on a background thread. You should change to viewmodel first and template out into pages.

Your immediate problem is caused by implicit expectations in the design that make your code fragile.