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
+      let prev_group = ''
-    endif
+    else
-
+      if i == 0
-    if prev_group != ''
+        let line .= '%#'.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
       endif
-    endif
 
-    if group != prev_group
+      if prev_group != '' && group != ''
-      let line .= '%#'.group.'#'
+        let line .= s:get_seperator(self, prev_group, group, side)
-    endif
+      endif
-    let line .= s:get_accented_line(self, group, contents)
 
-    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 = []

autoload/airline/extensions/tabline.vim

@@ -256,6 +256,7 @@ function! s:get_buffers()
 
   call b.add_section('airline_tabfill', '')
   call b.split()
+  call b.add_section('airline_tabfill', '')
   call b.add_section('airline_tabtype', ' buffers ')
 
   let s:current_bufnr = cur

autoload/airline/highlighter.vim

@@ -39,6 +39,16 @@ function! s:get_array(fg, bg, opts)
         \ : [ '', '', fg, bg, join(a:opts, ',') ]
 endfunction
 
+function! airline#highlighter#is_same_bg(group1, group2)
+  let color1 = airline#highlighter#get_highlight(a:group1)
+  let color2 = airline#highlighter#get_highlight(a:group2)
+  if has('gui_running')
+    return color1[1] == color2[1]
+  else
+    return color1[3] == color2[3]
+  endif
+endfunction
+
 function! airline#highlighter#get_highlight(group, ...)
   let fg = s:get_syn(a:group, 'fg')
   let bg = s:get_syn(a:group, 'bg')

t/builder.vim

@@ -13,9 +13,18 @@ describe 'active builder'
 
   it 'should transition colors from one to the next'
     call s:builder.add_section('Normal', 'hello')
-    call s:builder.add_section('NonText', 'world')
+    call s:builder.add_section('Search', 'world')
     let stl = s:builder.build()
-    Expect stl =~ '%#Normal#hello%#Normal_to_NonText#>%#NonText#world'
+    Expect stl =~ '%#Normal#hello%#Normal_to_Search#>%#Search#world'
+  end
+
+  it 'should reuse highlight group if background colors match'
+    highlight Foo1 ctermfg=1 ctermbg=2
+    highlight Foo2 ctermfg=3 ctermbg=2
+    call s:builder.add_section('Foo1', 'hello')
+    call s:builder.add_section('Foo2', 'world')
+    let stl = s:builder.build()
+    Expect stl =~ '%#Foo1#hello>world'
   end
 
   it 'should split left/right sections'
@@ -27,9 +36,9 @@ describe 'active builder'
   it 'after split, sections use the right separator'
     call s:builder.split()
     call s:builder.add_section('Normal', 'hello')
-    call s:builder.add_section('NonText', 'world')
+    call s:builder.add_section('Search', 'world')
     let stl = s:builder.build()
-    Expect stl =~ '%#Normal#hello%#Normal_to_NonText#<%#NonText#world'
+    Expect stl =~ 'hello%#Normal_to_Search#<%#Search#world'
   end
 
   it 'should not repeat the same highlight group'
@@ -66,9 +75,9 @@ describe 'inactive builder'
 
   it 'should transition colors from one to the next'
     call s:builder.add_section('Normal', 'hello')
-    call s:builder.add_section('NonText', 'world')
+    call s:builder.add_section('Search', 'world')
     let stl = s:builder.build()
-    Expect stl =~ '%#Normal_inactive#hello%#Normal_to_NonText_inactive#>%#NonText_inactive#world'
+    Expect stl =~ '%#Normal_inactive#hello%#Normal_to_Search_inactive#>%#Search_inactive#world'
   end
 
   it 'should not render accents'

t/extensions_default.vim

@@ -2,6 +2,7 @@ let g:airline_theme = 'dark'
 call airline#init#bootstrap()
 call airline#init#sections()
 source plugin/airline.vim
+call airline#load_theme()
 
 describe 'default'
   before