Enable unfolding 3 lines of context around each hunk.

README.mkd

@@ -213,7 +213,9 @@ See the customisation section below for how to change the defaults.
 
 ### Folding
 
-Use the `GitGutterFold` command to fold all unchanged lines, leaving just the hunks visible.  Use the command again to restore the previous view.
+Use the `GitGutterFold` command to fold all unchanged lines, leaving just the hunks visible.  Use `zo` to unfold 3 lines of context above and below a hunk.
+
+Execute `GitGutterFold` a second time to restore the previous view.
 
 
 ### Customisation

autoload/gitgutter/fold.vim

@@ -1,6 +1,7 @@
 function! gitgutter#fold#enable()
   call s:save_fold_state()
 
+  call s:set_fold_levels()
   setlocal foldexpr=gitgutter#fold#level(v:lnum)
   setlocal foldmethod=expr
   setlocal foldlevel=0
@@ -26,7 +27,32 @@ endfunction
 
 
 function! gitgutter#fold#level(lnum)
-  return gitgutter#hunk#in_hunk(a:lnum) ? 0 : 1
+  return gitgutter#utility#getbufvar(bufnr(''), 'fold_levels')[a:lnum]
+endfunction
+
+
+" A line in a hunk has a fold level of 0.
+" A line within 3 lines of a hunk has a fold level of 1.
+" All other lines have a fold level of 2.
+function! s:set_fold_levels()
+  let fold_levels = ['']
+
+  for lnum in range(1, line('$'))
+    let in_hunk = gitgutter#hunk#in_hunk(lnum)
+    call add(fold_levels, (in_hunk ? 0 : 2))
+  endfor
+
+  for lnum in range(1, line('$'))
+    if fold_levels[lnum] == 2
+      let pre = lnum >= 3 ? lnum - 3 : 0
+      let post = lnum + 3
+      if index(fold_levels[pre:post], 0) != -1
+        let fold_levels[lnum] = 1
+      endif
+    endif
+  endfor
+
+  call gitgutter#utility#setbufvar(bufnr(''), 'fold_levels', fold_levels)
 endfunction