ScrollViewer visibility bug in TextBox control

Description

The property VerticalScrollBarVisibility of the TextBox control doesn’t work as expected. Setting such property to Visible does not reveal the vertical scrollbar. Instead, it works like setting the property to Auto. So, for example, it is not possible to mimic Notepad interface where the vertical scrollbar is visible, but disabled, when there is a short amount of text.

Steps to Reproduce

1. Create a new WPF .Net Core app.
2. Add a Textbox to the Grid in MainWindow.xaml and set its property VerticalScrollBarVisibility to Visible.
3. Add the ModernWpf library to the project and then add <ui:XamlControlsResources /> in App.xaml.
4. Run the app.

Expected Behavior

The vertical scrollbar should be shown disabled if there is a short amount of text and it will be enabled when needed.

Actual Behavior

The vertical scrollbar is hidden and doesn’t show up until needed.

[Kinnara]

Below is how the same markup looks in WinUI. For a start I’ll try to make the behavior consistent with WinUI, although it also has its own issues.

[XamDR]

Ok, one last try

Instead of setting the textbox’s VerticalScrollBarVisibility, try setting ScrollViewer.VerticalScrollBarVisibility attached property on the textbox.

Edit: It should work in both cases. Are values other than visible working?

ScrollViewer.VerticalScrollBarVisibility doesn’t work either. The other values work just fine. I also noticed that the same issue happens with the horizontal scrollbar.