check background of groups to determine transition. resolves #599.

autoload/airline/builder.vim

@@ -23,36 +23,35 @@ function! s:prototype.build()
   let side = 1
   let prev_group = ''
   let line = ''
+  let i = 0
+  let length = len(self._sections)
 
-  for section in self._sections
+  while i < length
+    let section = self._sections[i]
     let group = section[0]
     let contents = section[1]
 
-    if group == '|'
+    if group == ''
+      let line .= contents
+    elseif group == '|'
       let side = 0
       let line .= contents
-      continue
-    endif
-
-    if prev_group != ''
-      if prev_group == group
-        let line .= side ? self._context.left_alt_sep : self._context.right_alt_sep
-      elseif group != ''
-        call airline#highlighter#add_separator(prev_group, group, side)
-        let line .= '%#'.prev_group.'_to_'.group.'#'
-        let line .= side ? self._context.left_sep : self._context.right_sep
+      let prev_group = ''
+    else
+      if i == 0
+        let line .= '%#'.group.'#'
       endif
-    endif
 
-    if group != prev_group
-      let line .= '%#'.group.'#'
-    endif
-    let line .= s:get_accented_line(self, group, contents)
+      if prev_group != '' && group != ''
+        let line .= s:get_seperator(self, prev_group, group, side)
+      endif
 
-    if group != ''
+      let line .= s:get_accented_line(self, group, contents)
       let prev_group = group
     endif
-  endfor
+
+    let i = i + 1
+  endwhile
 
   if !self._context.active
     let line = substitute(line, '%#.\{-}\ze#', '\0_inactive', 'g')
@@ -60,6 +59,19 @@ function! s:prototype.build()
   return line
 endfunction
 
+function! s:get_seperator(self, prev_group, group, side)
+  let line = ''
+  if airline#highlighter#is_same_bg(a:prev_group, a:group)
+    let line .= a:side ? a:self._context.left_alt_sep : a:self._context.right_alt_sep
+  else
+    call airline#highlighter#add_separator(a:prev_group, a:group, a:side)
+    let line .= '%#'.a:prev_group.'_to_'.a:group.'#'
+    let line .= a:side ? a:self._context.left_sep : a:self._context.right_sep
+    let line .= '%#'.a:group.'#'
+  endif
+  return line
+endfunction
+
 function! s:get_accented_line(self, group, contents)
   if a:self._context.active
     let contents = []