How to detect that Taskbar Thumbnail Preview is being drawn when DWMWA_HAS_ICONIC_BITMAP is set

Question

How to detect that Taskbar Thumbnail Preview is being drawn (because user is moving mouse over taskbar button) or is currently appearing on screen?

Why the code below does not make log entries when WM_DWMSENDICONICTHUMBNAIL msg should be arriving to form?

    procedure TStartInfoForm.FormCreate(Sender: TObject);
    begin        
      SetWindowAttribute(DWMWA_HAS_ICONIC_BITMAP);               
    end;
    function TStartInfoForm.SetWindowAttribute(dwAttr: DWORD): HRESULT;
    var
      Attr: function(hwnd: hwnd; dwAttribute: DWORD; pvAttribute: Pointer;
       cbAttribute: DWORD): HRESULT; stdcall;
     hndDLLHandle : THandle;
     EnableAttribute : DWORD;
    begin
      EnableAttribute := DWORD(true);
      try
        hndDLLHandle := loadLibrary('dwmapi.dll');
        if hndDLLHandle <> 0 then
        begin
          Attr := getProcAddress(hndDLLHandle, 'DwmSetWindowAttribute');
          if addr(Attr) <> nil then
           Result := Attr(Handle, dwAttr, @EnableAttribute, SizeOf(EnableAttribute));
        end;
      finally
        freeLibrary(hndDLLHandle);
      end;
    end;

procedure TStartInfoForm.WndProc(var Msg : TMessage);
begin    
 DebugNote(0,'wndproc '+inttohex(msg.Msg,3)+' w '+inttohex(Msg.wparam,3)+' l '+inttohex(Msg.lparam,3));
    
   case Msg.Msg of    
    WM_DWMSENDICONICTHUMBNAIL : 
   begin
    DebugNote(0,' iconiclive');
   end;
    WM_DWMSENDICONICLIVEPREVIEWBITMAP:
   begin
    DebugNote(0,' iconiclive2 ');    
   end;

      WM_WINDOWPOSCHANGING:
      begin
//   MessageBeep(1);
      end                ;
      WM_WINDOWPOSCHANGED:
      begin
//   MessageBeep(1);
      end
   end;
   inherited;// call this for default behaveour
 end;