Many people want/need the sidebar patch from mutt, but currently it implies USE=vanilla, which makes mutt useless (and full of errors). This bug is a todo for myself, and a progress checker for others to get USE=sidebar not imply USE=vanilla. I should push my mercurial tree somewhere, as well as its patchqueue, and get the sidebar patch in there to maintain it (and/or help others to).
I've got it working, but only when USE=-nntp. The only change needed was to rediff the buildsystem bits.
-r16 should do it. If the sidebar patch behaves (when not being configured to be in use) I might even switch it on by default and remove the USE-flag for it. All enjoy your feature-rich and bug-less Mutt with sidebar.