Here is an article which describes some of the design, and claims to have link to the source code, but the link just takes you to the download page to the installer. I downloaded and installed the msi in hopes that the source was bundled in with the installer, but no luck there. XML Notepad design article (no source code though)

There could be some side effect of the GetValueAsLong() method call that the rest of the system is depending on. If the code looks bad at face value, you should only assume it gets worse as you dig into it.