WPF ItemControls do not show all items - wpf

Question

What I'm doing is quite simple. I`m hosting list of UserControls in an ItemControl.
I made up a test application with a simple TextBox as a UserControl, it works as expected. But when I incorporate this in my main code, the ItemControl does not display all the Text Boxes. It always limits it to 3, and the values in the text boxes don`t get updated.
Is the IDE limiting how many UserControls I can view in the designer when I have other controls? When I execute the code, I do see all the UserControls. I just need to show 6 and not 100s.I would like to see all the UserControls in the designer in my main code and not just the test application.
The code between the test application and my main application is the same.
Here is the code, I`m attaching the pictures of what I see in the Designer.
/// <summary>
/// View Model Class for the MainWindow.
/// </summary>
class AViewModel : INotifyPropertyChanged
{
public AViewModel()
{
ZList = new ObservableCollection<StringObject>
{
new StringObject {Value = "1"},
new StringObject {Value = "2"},
new StringObject {Value = "3"},
new StringObject {Value = "4"},
new StringObject {Value = "5"},
new StringObject {Value = "6"},
new StringObject {Value = "7"},
};
}
private ObservableCollection<StringObject> zlist;
public ObservableCollection<StringObject> ZList
{
get { return zlist; }
set
{
zlist = value;
OnPropertyChanged("ZList");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
}
public class StringObject
{
public string Value { get; set; }
}
}
XML of Main Window
<Window x:Class="IssueWPF.MainWindow"
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:IssueWPF"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525">
<Window.DataContext>
<local:AViewModel></local:AViewModel>
</Window.DataContext>
<Grid>
<Viewbox>
<ItemsControl ItemsSource="{Binding ZList}" Height="220" Margin="0">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Value}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Vertical" IsItemsHost="True" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
</ItemsControl>
</Viewbox>
</Grid>
</Window>