Sub-Pages only of menu items in current branch using a custom menu

Using the plugin “Advanced menu widget” by Ján Bočínec one can get pretty close. With one big exception: when you look at a post from category A, the whole sub-menu vanishes (except for a remaining empty <ul></ul>-construct).

So we are still looking for a working solution. We are going to try it now with Gecka submenu plugin and tons of CSS. But I am afraid that even this plugin does not provide enough CSS classes.

Thus this is half an answer 😉