RadioButtons: Fix selected radio button not being focused on tab navigation when first element in TAB navigation

[Felix-Dev]

Description

This PR fixes issue #3021 where focus was not set to the selected radio button when re-entering the specific RadioButtons control via TAB navigation. This issue occurs when the RadioButtons control is the first control in the TAB navigation route. In that case, GettingFocusEventArgs.OldFocusedElement() returns null, even if the focus was indeed set to another item before (in this case the last item in the TAB navigation route). With this PR, we now also set focus on the selected radio button (if any) when there is no old focused element as per GettingFocusEventArgs.

In order to properly test this case I had to create a new test page without the Back button and ToggleTheme button typically available on a test page as those buttons come first in the TAB navigation route so GettingFocusEventArgs.OldFocusedElement() would not be null upon tabbing into the RadioButtons control. I did not want to remove those two buttons for the existing RadioButtons test page so my only option was to create a new test page here.

I have modified an existing interaction test - FocusComingFromAnotherRepeaterTest() - to cover this specific case. The modified test is not an exact 1-to-1 re-build as I removed checks like Verify.IsTrue(item3.IsSelected); for easier testing. This should be fine as those checks are covered in the existing SelectionTest().

Motivation and Context

Fixes #3021.

How Has This Been Tested?

Tested visually and added interaction test.

Screenshots:

(The new landing page for RadioButtons tests)

(The new special focus test page)

[StephenLPeters]

[StephenLPeters]

looks good, Although its seems weird to me that GettingFocusEventArgs.OldFocusedElement() returns null when focus wraps. @Austin-Lamb do you know why this is the case?

[StephenLPeters]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

@Felix-Dev do you happen to know if this also reproduces when RadioButtons is the last focusable elmement in the tree and you use Shift+Tab to move focus from the first element to the last element (the radiobuttons).

[Felix-Dev]

Yep, same issue with the other way around (GettingFocusEventArgs.OldFocusedElement() = null):

(And yes, the workaround works here as well, as expected.)

If this is indeed a bug in GettingFocusEventArgs, it makes sense to add such a comment above the workaround referencing your tracking issue #3066.

[Felix-Dev]

@StephenLPeters See the gif above (I pinged you previously but that was in error, I posted the wrong gif. This ping is just meant to infomr you that the correct gif is now up above.)

[StephenLPeters]

That new issue has a reference to this check in so I don't think a comment is necessary.