What problem does mf::Shell::handle_surface_created() actually solve?

[Alan Griffiths]

In my trawl through the data model I came across
handle_surface_created() and wondered exactly what problem it solves.

I suspect the "scene" rework will offer a better solution, but I'd like
to understand what's going on before proposing anything.

Firstly, note that *this function is not the subject of any tests*. So
changes that break the required behaviour will not cause test failures.

Now, from mf::SessionMediator::create_surface()

            // TODO: NOTE: We use the ordering here to ensure the shell
acts on the surface after the surface ID is sent over the wire.
            // This guarantees that notifications such as, gained focus,
etc, can be correctly interpreted by the client.
            // To achieve this order we rely on done->Run() sending
messages synchronously. As documented in mfd::SocketMessenger::send.
            // this will require additional synchronization if
mfd::SocketMessenger::send changes.

And the active code is:

    void
    ms::SessionManager::handle_surface_created(std::shared_ptr<mf::Session>
    const& session)
    {
        auto shell_session =
    std::dynamic_pointer_cast<msh::Session>(session);

        set_focus_to(shell_session);
    }

So, it appears to be a mechanism for setting the focus to the session
that owns a surface /after/ the surface data has been sent to the
client. Except that we also call it from mf::SurfaceId
ms::SessionManager::create_surface_for()

    // TODO: We use this to work around the lack of a
    SessionMediator-like object for internal clients.
    // we could have an internal client mediator which acts as a factory
    for internal clients, taking responsibility
    // for invoking handle_surface_created.

In any case, the requirement appears to be that to focus on a session
immediately a new surface in the session is known to the client.

I assume this is so that input can be routed to the surface before it is
visible? (That surface hasn't yet posted a frame so it is not available
to be rendered.)
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