Normally the graphics window is re-drawn whenever the window is uncovered after being covered by another window. This generally works, but does not work by default with Silicon Graphics X-servers. (This may also be a problem with other X-servers, but at present this has not occurred.)
The problem is that Silicon Graphics X-servers do not provide ``backing store'' by default. FIT2D requests ``backing store'' when the graphics window is created, and this works fine on all other known X-servers. When ``backing store'' is enabled it is the window manager which automatically re-draws the window when it is uncovered.
To make this work for Silicon Graphics X-servers:
The file /usr/lib/X11/xdm/Xservers (which is used when launching Xsgi) should NOT contain the option -bs. (see man Xsgi).
This will also avoid the same problem for IDL and other programs.
Given the cheapness and availability of RAM there is no good reason for not providing ``backing store''.
(Dominique Bourgeois can be thanked for investigating and finding the solution to this annoying problem.)