Yeah, mouse support is an undocumented feature and not officially supported. It's there, likely for the program editor, except zep did not meant to put it in the docs and/or anyone to use it.
Also you should add options that allows people to hide title bars or icons or even disable drag and drop. That could be handy if for example someone was to use this in a JRPG and didn't want the mouse stuff.
I think you're better drawing rectangles in that case. Unless you need to move windows, that is.