patch 8.1.1456: WinBar not redrawn after scrolling one line

Problem:    WinBar not redrawn after scrolling one line.
Solution:   Exclude the winbar height when deciding what to redraw.
            (closes #4473)
diff --git a/src/testdir/test_winbar.vim b/src/testdir/test_winbar.vim
index a34c29e..c0b43b9 100644
--- a/src/testdir/test_winbar.vim
+++ b/src/testdir/test_winbar.vim
@@ -110,3 +110,15 @@
 
   bwipe!
 endfunc
+
+func Test_redraw_after_scroll()
+  new
+  amenu 1.10 WinBar.Next :let g:did_next = 11<CR>
+  redraw
+  call assert_equal("  Next", Screenline(1))
+  echo "some\nmore"
+  redraw
+  call assert_equal("  Next", Screenline(1))
+  bwipe!
+endfunc
+