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I've created a patch that adds another menu_block configuration setting.
Basically when you set Starting level to "Children of active menu", it allows you to choose whether or not you still want the menu to show at end-of-the-line items, albeit with their siblings as links.
Link 1 Link 2 Link A Link i Link ii Link iii Link iv Link v Link B Link 3
When navigating to 1 > A > iv, the menu would normally disappear.
With patch in comment #1 and the toggle enabled, it will show:
Link A --------- Link i Link ii Link iii *Link iv* Link v
Seeing as I needed this functionality myself, I can already assure you that this patch works nicely.
I just thought it could contribute to this awesome module :)
P.S.: I think this issue may be related to and/or fixand .