Tab navigation is broken when RadioButtons is first Control in visual tree

[jwmsft]

Describe the bug
When RadioButtons is the first control in the visual tree, Tab navigation does not behave as described in the documentation.

1. Initial focus is set to the second control in the visual tree, not to the first RadioButtons control.
2. When Tab focus reaches the first RadioButtons control, the first radio button in the list always gets focus, even when another option is selected.

Steps to reproduce the bug
Create a blank UWP project and add this XAML.

<Page ... >

    <StackPanel>
        <muxc:RadioButtons Header="Options A">
            <x:String>Option 1</x:String>
            <x:String>Option 2</x:String>
            <x:String>Option 3</x:String>
        </muxc:RadioButtons>

        <muxc:RadioButtons Header="Options B">
            <x:String>Option 4</x:String>
            <x:String>Option 5</x:String>
            <x:String>Option 6</x:String>
        </muxc:RadioButtons>
    </StackPanel>
</Page>

Issue 1 Steps to reproduce:

1. Run the app. Focus is not set.
2. Press the Tab key.

Expected behavior
Initial focus is set to the first radio button in the first group, Option 1.
Actual behavior
Focus is set to the second control in the visual tree; in this case, the first radio button in the second group, Option 4.

Issue 2 Steps to reproduce:

1. Select an option in the first group (e.g - Option 3).
2. Select an option in the second group (e.g - Option 5).
3. Press Tab several times to cycle between the groups.

Expected behavior
Per documentation, "When a radio button is selected and a user tabs into the list, the selected radio button gets focus."
Actual behavior
When a user tabs into the first group, the first option always gets focus, regardless of whether an item is selected.

Version Info

NuGet package version:
[Microsoft.UI.Xaml 2.4.2]
[Microsoft.UI.Xaml 2.5.0-prerelease.200708003]

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
May 2020 Update (19041)	Yes
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Additional context

[Felix-Dev]

I would like take a look into this.

[Felix-Dev]

@jwmsft

Initial focus is set to the second control in the visual tree, not to the first RadioButtons control.

In your repro code, Focus is in the first RadioButtons control ("Option A"). While you don't see a focus rectangle, you can press the Space key to select the first item of "Option A" ("Option 1") or you can press the down arrow key to select the remaining items in "Option A".

I am working on a fix for issue 2.