xrandr cropping my window

[David Tilbrook]

Preamble:

% cvt 1600 1280
# 1600x1280 59.92 Hz (CVT 2.05M4) hsync: 79.51 kHz; pclk: 171.75 MHz
Modeline "1600x1280_60.00"  171.75  1600 1712 1880 2160  1280 1283
1290 1327 -hsync +vsync

Running the following:

xrandr --auto
xrandr --newmode "1600x1280_60.00"  171.75  1600 1712 1880 2160  1280
1283 1290 1327 -hsync +vsync
xrandr --addmode VGA1 "1600x1280_60.00"
xrandr --output VGA1 --mode "1600x1280_60.00"
 yields:

Default gamma for IRGB image is  2.20
  Compressing colormap ...no improvment [sic]
  Clipping image...(Adding border)...done
  Building XImage...done

It does accomplish most of what I want, but indeed added a black border
at the top of the display.

The root window has geometry 1600x1280+0+0, however the area
that's visible on the display is approximately 1520x1200.

This means that when maximizing a window, the bottom and left edges
are not displayed.

Are there adjustments I can make to get my full screen?

-- david