Thumb does not get shown in custom control

[DemonExposer]

Describe the bug

When creating a custom UserControl with a Canvas and a Thumb, the Thumb does not get shown

To Reproduce

In https://github.com/AvaloniaUI/AvaloniaUI.QuickGuides/blob/main/CustomControl/Controls/CustomUserControl.axaml, replace the TextBlock with the following:

<Canvas Background="Yellow" Width="200" Height="100">
	<Thumb Width="10" Height="10"
           Background="Blue"
           BorderBrush="Red"
           BorderThickness="3"
           Canvas.Left="75"
           Canvas.Top="50" />
</Canvas>

Now, the Canvas shows but the Thumb doesn't.
When inspecting with DevTools, it shows the Thumb in the visual tree, but it does not render it

Expected behavior

A Thumb should be shown on the Canvas

Avalonia version

11.2.4

OS

Windows

Additional context

No response

[maxkatz6]

Thumb is a primitive that doesn't have its own template by default

[DemonExposer]

@maxkatz6 okay, so then it's not a bug, but still, how would you put a thumb into a custom UserControl? Because I couldn't find any information on this

[timunie]

You need to add a ControlTemplate preferred via Style or ControlTheme. Check out existing ones from source if you have no idea how to. I believe Scrollbar is using one.

https://github.com/AvaloniaUI/Avalonia/blob/master/src%2FAvalonia.Themes.Fluent%2FControls%2FScrollBar.xaml#L21-L36

[DemonExposer]

Thanks for the help guys, I figured it out now!