diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 5ee22e2..1deb238 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -4,6 +4,9 @@
 
 func Test_diff_fold_sync()
   enew!
+  let g:update_count = 0
+  au DiffUpdated * let g:update_count += 1
+
   let l = range(50)
   call setline(1, l)
   diffthis
@@ -29,6 +32,9 @@
   call win_gotoid(winone)
   call assert_equal(23, getcurpos()[1])
 
+  call assert_equal(1, g:update_count)
+  au! DiffUpdated
+
   windo diffoff
   close!
   set nomodified