How to use IFrameworkInputPaneHandler when program runs under elevated privileges

Question

I'm having problems when a program that should receive notifications about Input Pane (the on-screen keyboard showed by TabTip.exe on Windows) runs under elevated privileges. In this situation, no notifications are received.

My code implements IFrameworkInputPane and calls AdviseWithHWND to get notified when the Input Pane is showing or hiding. I tried to run TabTip.exe under elevated privileges too, but even in this case, my program receives no notification about the status of the Input Pane. Here is my sample code. When running under elevated privileges (which is necessary in my real case), methods Hiding and Showing are never called:

' Code for test. Just Add a button and a Textbox to the Form

Imports System.ComponentModel
Imports System.Runtime.InteropServices

Public Class Form1

    ' Variables ----------------------------------------------------------
    Private moPaneHandler As FrameworkInputPaneHandler
    Private moFrameworkInputPane As IFrameworkInputPane
    Private miCookie As Integer

    ' Definitions --------------------------------------------------------
    Public Enum HRESULT As Integer
        S_OK = 0
        S_FALSE = 1
        E_NOINTERFACE = &H80004002
        E_NOTIMPL = &H80004001
        E_FAIL = &H80004005
        E_UNEXPECTED = &H8000FFFF
        E_OUTOFMEMORY = &H8007000E
    End Enum

    Public Structure RECT
        Public Left As Integer
        Public Top As Integer
        Public Right As Integer
        Public Bottom As Integer
        Public Sub New(
                  ByVal Left As Integer,
                  ByVal Top As Integer,
                  ByVal Right As Integer,
                  ByVal Bottom As Integer)
            Me.Left = Left
            Me.Top = Top
            Me.Right = Right
            Me.Bottom = Bottom
        End Sub
    End Structure


   <ComImport(), Guid("D5120AA3-46BA-44C5-822D-CA8092C1FC72")
    Public Class FrameworkInputPane
    End Class

    <ComImport()>
    <Guid("226C537B-1E76-4D9E-A760-33DB29922F18")>
    <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>
    Public Interface IFrameworkInputPaneHandler
        Function Showing(ByRef prcInputPaneScreenLocation As RECT,
                         ByVal fEnsureFocusedElementInView As Boolean) As HRESULT
        Function Hiding(ByVal fEnsureFocusedElementInView As Boolean) As HRESULT
    End Interface

    <ComImport()>
    <Guid("5752238B-24F0-495A-82F1-2FD593056796")>
    <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>
    Public Interface IFrameworkInputPane

        Function Advise(ByVal pWindow As IntPtr,
                        ByVal pHandler As Object,
                        ByRef pdwCookie As Integer) As HRESULT

        Function AdviseWithHWND(ByVal hwnd As IntPtr,
                                ByVal pHandler As IFrameworkInputPaneHandler,
                                ByRef pdwCookie As Integer) As HRESULT

        Function Unadvise(ByVal dwCookie As Integer) As HRESULT
        Function Location(ByRef prcInputPaneScreenLocation As RECT) As HRESULT
    End Interface

    Public Class FrameworkInputPaneHandler
        Implements IFrameworkInputPaneHandler

        Public Function Hiding(ByVal fEnsureFocusedElementInView As Boolean) As HRESULT Implements IFrameworkInputPaneHandler.Hiding
            With Form1.Textbox1
                .Text = .Text & Now & " - "
                .Text = .Text & "Hiding" & vbCrLf
            End With

            Return HRESULT.S_OK
        End Function

        Public Function Showing(ByRef prcInputPaneScreenLocation As RECT, ByVal fEnsureFocusedElementInView As Boolean) As HRESULT Implements IFrameworkInputPaneHandler.Showing
            With Form1.Textbox1
                .Text = .Text & Now & " - "
                .Text = .Text & " Showing "
                .Text = .Text & "   Top: " & prcInputPaneScreenLocation.Top
                .Text = .Text & "   Left: " & prcInputPaneScreenLocation.Left
                .Text = .Text & "   Height: " & prcInputPaneScreenLocation.Bottom - prcInputPaneScreenLocation.Top
                .Text = .Text & "   Width: " & prcInputPaneScreenLocation.Right - prcInputPaneScreenLocation.Left & vbCrLf
            End With

            Return HRESULT.S_OK
        End Function
    End Class

    Private Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
        Dim slRetCode As HRESULT = HRESULT.E_FAIL
        Dim stRect As RECT = New RECT()

        miCookie = 0
        moPaneHandler = New FrameworkInputPaneHandler()
        moFrameworkInputPane = New FrameworkInputPane

        If (moFrameworkInputPane IsNot Nothing) Then

            slRetCode = moFrameworkInputPane.AdviseWithHWND(Me.Handle, moPaneHandler, miCookie)

            If slRetCode = HRESULT.S_OK Then
                Textbox1.Text = Textbox1.Text & "Advise OK! Cookie " & miCookie & vbCrLf
            Else
                Textbox1.Text = Textbox1.Text & "Advise Error " & slRetCode
            End If
        End If
    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load

        Button1.Text = "Advise Input Pane"
    End Sub

    Private Sub Form1_Closing(sender As Object, e As CancelEventArgs) Handles Me.Closing
        If miCookie <> 0 Then
            Call moFrameworkInputPane.Unadvise(miCookie)
        End If

        moPaneHandler = Nothing
    End Sub
End Class

Everything works fine if the program runs "de-elevated". Does anybody has an idea on how to make this work?
Thank you.