Replaced Java Viewer 2.6.2 with 2.7.2. Page specifies WIDTH="800" HEIGHT="480", ShowControls=no, ScalingFactor=100.
The viewer draws a window 800x480 with scrollbars, so the actual viewport is smaller than the actual screen. The scrollbars must be used to see portion of the screen obscured by the scrollbars.
This did not happen with the earlier viewer, the display was a borderless image exactly 800x480.