[Windows] Register focus events for controls that can receive focus

[MartyIX]

Description of Change

I noticed that MAUI registers GotFocus and LostFocus events for all platform views on Windows.

However, for MAUI <Label>s, I believe it's not really necessary.

It seems that TextBlock (WinUI 3 type that MAUI's Label is mapped to) can be focusable as it contains this method:

// Focus and selection handling related overrides and helpers.
bool IsFocusable() final
{
    return IsActive() &&
        IsVisible() &&
        IsEnabled() && (m_isTextSelectionEnabled || IsTabStop()) &&
        AreAllAncestorsVisible();
}

However, support for TabStops was removed in #1777 (see also #1646 for reasoning) and MAUI does not support text selection for Labels.

Performance impact

• MAIN 1722247190..speedscope.MAIN.json
• PR 1722247261..speedscope.PR.json

-> ~ 96% improvement

Issues Fixed

Contributes to #21787

Alternative approaches

There is FocusManager in WinUI and it has similar events ( https://learn.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.input.focusmanager?view=windows-app-sdk-1.5#events), so instead of registering events on all views one by one (and unregistering for that matter). One can probably subscribe FocusManager events once and just check if the platform view corresponds to one in a dictionary and do the action.

[jonathanpeppers]

@PureWeen who would know the most about accessibility here? Is there some way focus is used for screen-readers?

[jonathanpeppers]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[MartyIX]

Replaced with #24355