I found 2 little things.....

Creating a normal CL_MainWindow, i have noticed that the viewport coordinates of display window don't match correctly.
Or the CL_Draw::box method don't work correctly.
To create a box touching all borders of the window (with a client area of 640x480), i need to set left,top,right,bottom with 0,1,639,480.
And the right-top pixel of the box (cross of top and right box's border) is missing.....
No matter of box's size: pixel is always missing.