Current neomutt sidebar patch does change COLS which does
not work with threadsafe ncurses6. Beside this: do not
bother other non-sidebar users as well make sidebar delimeter
look well.

---
 configure.ac |    8 ++++++++
 sidebar.c    |    9 ++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

--- configure.ac
+++ configure.ac	2016-06-13 09:03:34.248832759 +0000
@@ -312,6 +312,14 @@ main ()
                 if test x$mutt_cv_curses != x/usr; then
                         LDFLAGS="$LDFLAGS -L${mutt_cv_curses}/lib"
                         CPPFLAGS="$CPPFLAGS -I${mutt_cv_curses}/include"
+                fi
+                if test -d /usr/include/ncurses5 ; then
+                        if test -d /usr/lib64/ncurses5 ; then
+                            LDFLAGS="$LDFLAGS -L/usr/lib64/ncurses5"
+                        else
+                            LDFLAGS="$LDFLAGS -L/usr/lib/ncurses5"
+                        fi
+                        CPPFLAGS="$CPPFLAGS -I/usr/include/ncurses5"
                 fi])
 
 	AC_CHECK_FUNC(initscr,,[
--- sidebar.c
+++ sidebar.c	2016-06-13 09:10:18.201095164 +0000
@@ -651,6 +651,7 @@ static int draw_divider (int first_row,
 {
   /* Calculate the width of the delimiter in screen cells */
   int delim_len = mutt_strwidth (SidebarDividerChar);
+  int altchar = 0;
 
   if (delim_len < 1)
     return delim_len;
@@ -661,13 +662,19 @@ static int draw_divider (int first_row,
   if (delim_len > SidebarWidth)
     return -1;
 
+  if (delim_len == 1 && strcmp(SidebarDividerChar, "|") == 0)
+    altchar++;
+
   SETCOLOR(MT_COLOR_DIVIDER);
 
   int i;
   for (i = 0; i < num_rows; i++)
   {
     move (first_row + i, SidebarWidth - delim_len);
-    addstr (NONULL(SidebarDividerChar));
+    if (altchar)
+      addch (ACS_VLINE);
+    else
+      addstr (NONULL(SidebarDividerChar));
   }
 
   return delim_len;