A full screen window is not always desirable.  If the user is working on a
large, scrolling screen, they may only want a window the size of the
visible display.  The following example calculates the visible area on a
screen and opens a window in that area.  The example assumes that the
screen display clip is as large or larger than text overscan (OSCAN_TEXT)
which is set by the user.  The window is opened in the text overscan area,
not within the actual display clip that is used for the screen.  Use
QueryOverscan() to find the standard overscan rectangles (display clips)
for a screen.  Use the graphics library call VideoControl() to find the
true display clip of the screen (see the chapter on "Graphics Primitives"
for more information on VideoControl()).  The ViewPortExtra structure
contains the display clip information.

    About Screen Coordinates.
    -------------------------
    The screen's actual position may not exactly equal the coordinates
    given in the LeftEdge and TopEdge fields of the Screen
    structure.  This is due to hardware constraints that limit the
    fineness of the positioning of the underlying constructs.  This may
    cause a window which is opened in the visible part of the screen to
    be incorrectly positioned by a small number of pixels in each
    direction.  See the discussion of the screen's LeftEdge and
    TopEdge in the "Intuition Screens" chapter for more information.

     visiblewindow.c 

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