WPF Modal Window Disables Scrollbars (ScrollViewer)

Question

The code below creates a working scrollbar if the Window host is non-modal using Show();

The code below creates a non-working scrollbar if the Window host is in Modal Mode using ShowDialog();

Incidentally even using the Xceed.Wpf.Toolkit.MessageBox instead of a normal WPF window results in creating a non-working scrollbar. So the issue seems to stem from the Modal Window state.

This post from almost a decade ago reports the same issue! Modal windows don't support scroll: wpf scrollviewer dialogbox.

I tried setting the ScrollViewer and StackPanel to the same height, that failed to make a difference. I tried compiling in 4.8 (vs 4.0) that also failed to make a difference. I'm open to other workarounds as well because this issue seems systemic. My intent is to show a Modal Window when an exception is raised, pausing the main thread in the application until the Modal Window exits.

    var myScrollViewer = new ScrollViewer();
    var Stacker = new StackPanel();

    myScrollViewer.HorizontalScrollBarVisibility = ScrollBarVisibility.Auto;
    myScrollViewer.Height = 360;
    Stacker.Height = 360;
    Stacker.Children.Add(Text_Title);
    Stacker.Children.Add(Paragraph_X);

    // Add the StackPanel as the lone child of the ScrollViewer
    myScrollViewer.Content = Stacker;

Answer 1

You could implement it in Xaml. When the height of the StackPanel is greater than the height of the ScrollViewer, the ScrollBar works. If I misunderstood, please let me know.
ModalWindow.xaml:

<Window x:Class="SaveFileFromMultipleTabItems.Window1"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
        xmlns:local="clr-namespace:SaveFileFromMultipleTabItems" WindowStartupLocation="CenterScreen"  
        mc:Ignorable="d"  
        Title="Window1" Height="360" Width="400">  
    <ScrollViewer x:Name="myScrollViewer" Height="360">  
        <StackPanel x:Name="Stacker" Height="460">  
            <Button Click="Button_Click" Content="click"/>  
            <Button Click="Button_Click" Content="click"/>  
            <RichTextBox Width="200" Height="100"   FontSize="40">  
                <FlowDocument>  
                    <Paragraph >  
                        <Run >Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1</Run>  
                    </Paragraph>  
                    <Paragraph >  
                        <Run >Paragraph 2,Paragraph 2,Paragraph 2,Paragraph 2,Paragraph 2</Run>  
                    </Paragraph>  
                </FlowDocument>  
            </RichTextBox>  
            <RichTextBox Width="200" Height="100"   FontSize="40">  
                <FlowDocument>  
                    <Paragraph >  
                        <Run >Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1,Paragraph 1</Run>  
                    </Paragraph>  
                    <Paragraph >  
                        <Run >Paragraph 2,Paragraph 2,Paragraph 2,Paragraph 2,Paragraph 2</Run>  
                    </Paragraph>  
                </FlowDocument>  
            </RichTextBox>  
            <TextBox Width="200" Height="50" Text="hello" TextWrapping="Wrap" FontSize="40"/>  
            <TextBox Width="200" Height="50" Text="hello" TextWrapping="Wrap" FontSize="40"/>  
        </StackPanel>  
    </ScrollViewer>  
</Window>  

Modal.xmal.cs:

 public partial class Window1 : Window  
  {  
    public Window1()  
    {  
      InitializeComponent();  
    }  
    private void Button_Click(object sender, RoutedEventArgs e)  
    {  
      MessageBox.Show("hello");  
    }  
  }  

MainWindow.xaml:

 <StackPanel >  
            <Button Content="open modal window" Height="40" Click="Button_Click"/>  
            <Button Content="click" Height="40" Click="Button_Click_1"/>  
        </StackPanel>  

MainWindow.xaml.cs:

public partial class MainWindow : Window  
  {  
    public MainWindow()  
    {  
      InitializeComponent();  
    }  
    private void Button_Click(object sender, RoutedEventArgs e)  
    {  
      Window1 window = new Window1();  
      window.ShowDialog();  
    }  
    private void Button_Click_1(object sender, RoutedEventArgs e)  
    {  
      MessageBox.Show("hello");  
    }  
  }  

The result:

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