I've rebuilt the interpreter using a version of that library that does
not exhibit the problem:
Intel Mac OS X 10.5 and higher:
http://sourceforge.net/projects/qtads/files/qtads-2.x/2.0.1/QTads-2.0.1-scrollfix-intel.dmg/download
PowerPC Mac OS X 10.5 and higher:
http://sourceforge.net/projects/qtads/files/qtads-2.x/2.0.1/QTads-2.0.1-scrollfix-ppc.dmg/download
Note that this was a Mac-specific problem; if you're not using the Mac
version of the interpreter, then you're not affected.