The menu is functionally completely broken. Because panel visibility is determined purely by :hover or :focus, as soon as you try to click on an item in the panel, that focus is lost and the panel disappears, taking your attempted click with it.
There are only three approaches that you can reasonably use here (they can be combined, too):
① Labels, which allow some rather nifty structure-breaking tricks on :hover;
② Change the .bmenu > a things from links (because you can’t nest links), and put the panel contents inside that node, then use :hover;
When you hover over an item in the main list, then move your mouse to the right, does the sub-menu stay the same or revert to the sub-menu for the first entry? (EDIT: Looks like that bug has been fixed. Earlier this morning, it would revert to the first sub-menu, now it works as expected)
There are only three approaches that you can reasonably use here (they can be combined, too):
① Labels, which allow some rather nifty structure-breaking tricks on :hover;
② Change the .bmenu > a things from links (because you can’t nest links), and put the panel contents inside that node, then use :hover;
③ Use radio buttons and :checked.