Zoom < 100% is now implemented in version 2.8.6 (in the form of a fraction 1/n, n=1,2,...,8 - a trivial change would even allow going beyond, but I guess 1/8 is already sufficient for most purposes). Version 2.8.6 also contains many other enhancements, as well as several bug corrections, including one bug which probably existed since the first days of xpaint and caused random crashes.