Improve NumericUpDown and AutoCompleteBox focusability

[mgnslndh]

What does the pull request do?

Makes the NumericUpDown control focusable. Focus is delegated its child TextBox.

What is the current behavior?

NumericUpDown is not focusable.

What is the updated/expected behavior with this PR?

Makes it possible to focus the NumericUpDown control and when that happens focus is delegated to the containing TextBox control.

How was the solution implemented (if it's not obvious)?

• Override Focusable property default value to true
• Focus the TextBox when NumericTextBox gets focus.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

I tried adding unit tests for this behavior but failed. It boils down to the visual tree not being setup correctly during test so the calls to TextBox.Focus() will fail under test, but works in a normal app scenario.

If it is possible to make a working test then please advise me how to proceed. The RealFocus test service is not enough for this test case.

Breaking changes

Don't think so.

Fixed issues

Fixes #8069
Fixes #13339
Fixes #11152
Fixes #5016
Fixes #12921

[maxkatz6]

Does it work with Shift+Tab or any other way of back-focus to previous focusable element?

[mgnslndh]

@maxkatz6

Does it work with Shift+Tab or any other way of back-focus to previous focusable element?

No, the PR does not address that scenario. I'll look into it.

[IanRawley]

I've used the ideas in this as a base to manage something for a basic control I'm working on, and the back navigation aspect can be handled to an extent with

if (e.Source == this && e.KeyModifiers.HasFlag(KeyModifiers.Shift))
{
    KeyboardNavigationHandler.GetNext(this, NavigationDirection.Previous)?.Focus(NavigationMethod.Tab);
}

in the OnGotFocus override.

[IanRawley]

I've refactored my version to account for a few oversights, and sometimes overthinking things, and come to this version.

protected override void OnGotFocus(GotFocusEventArgs e)
{
    base.OnGotFocus(e);
    if (e.Source == this)
    {
        if (e.KeyModifiers.HasFlag(KeyModifiers.Shift))
            KeyboardNavigationHandler.GetNext(this, NavigationDirection.Previous)?.Focus(NavigationMethod.Tab, e.KeyModifiers);
        else
            TextBox?.Focus(NavigationMethod.Tab);

        e.Handled = true;
    }
}

I'm not sure what you'd need to change to account for the ButtonSpinner that wraps the TextBox in the Simple Theme. I suspect nothing actually, but haven't looked into it.