Fix: UIA tree not visible for Avalonia content embedded via WinFormsAvaloniaControlHost

[AndreyPolovodov]

What does the pull request do?

Fixes the UIA accessibility tree being invisible for Avalonia content hosted in WinForms/WPF via WinFormsAvaloniaControlHost. Tools like Inspect.exe, Accessibility Insights for Windows, Narrator and FlaUI see the embedded Avalonia HWND as an opaque [pane] with no children.

What is the current behavior?

EmbeddableControlRootAutomationPeer implements IEmbeddedRootProvider but not IRootProvider. In the Win32 backend, AutomationNode.Create only checks for IRootProvider to instantiate RootAutomationNode:

else if (peer.GetProvider<IRootProvider>() is not null)
    return new RootAutomationNode(peer);
else
    return new AutomationNode(peer);

Embedded roots fall into the last branch and get a plain AutomationNode that doesn't implement IRawElementProviderFragmentRoot and returns null from GetHostRawElementProvider (no UiaHostProviderFromHwnd call). UiaReturnRawElementProvider then responds to WM_GETOBJECT with E_FAIL (0x80004005) and the Avalonia subtree is missing from UIA.

What is the updated/expected behavior with this PR?

EmbeddableControlRootAutomationPeer also implements IRootProvider. AutomationNode.Create picks the RootAutomationNode path, UiaReturnRawElementProvider succeeds, and the embedded Avalonia tree is fully exposed to UIA clients.

Verified by hosting an Avalonia control in a WPF window through WindowsFormsHost -> WinFormsAvaloniaControlHost and inspecting it with Inspect.exe and Accessibility Insights for Windows: the full control tree is now reachable instead of a single [pane].

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

IRootProvider and IEmbeddedRootProvider share GetFocus, GetPeerFromPoint and FocusChanged with identical signatures, so the existing implementation satisfies both. The only extra IRootProvider member, PlatformImpl, is added as an explicit interface implementation forwarding to Owner.PlatformImpl.

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

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065734-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[AndreyPolovodov]

@cla-avalonia agree

[MrJul]

IRootProvider makes sense on EmbeddableControlRoot despite the name: that is a "true" root, i.e. it's backed by a ITopLevelImpl.

LGTM!