cat (noskin) is broken, categories are not correctly selected.
http://demo3.b2evolution.net/stable/index.php?cat=3
Forwards to http://demo3.b2evolution.net/stable/blog1.php/background/
demo3.b2evolution.net/stable/blog1.php?cat=-3
Forwards to http://demo3.b2evolution.net/stable/blog1.php/background/
According to the documentation cat=-3 is supposed to show all categories expect Background (and the children of Background).
This broken also exists in the latest version of 4.x
Thanks @davidnewcomb,
This issue is now resolved and will be included in the next release of b2evolution.