test for warning sections enabled in active splits.

t/extensions_default.vim

@@ -1,5 +1,6 @@
 call airline#init#bootstrap()
 call airline#init#sections()
+source plugin/airline.vim
 
 describe 'default'
   before
@@ -19,5 +20,12 @@ describe 'default'
     Expect stl =~ 'airline_x_to_airline_z'
     Expect stl =~ 'airline_z_to_airline_y'
   end
+
+  it 'should only render warning section in active splits'
+    wincmd s
+    Expect getwinvar(1, '&statusline') =~ 'whitespace'
+    Expect getwinvar(2, '&statusline') !~ 'whitespace'
+    wincmd c
+  end
 end