A USE flag to toggle if a double click on the titlebar of a window maximizes or shades this window. Reproducible: Always Steps to Reproduce:
http://fluxbox-wiki.org/index.php/Howto_Make_dblclick_titlebar_maximize
Ouch, this should be requested upstream as a runtime configure option. Well, maintainer's decision.
This is configurable at run-time in the latest git sources, and a new fluxbox release is scheduled "soon". Please hang on, if you can :)