patch 8.1.2363: ml_get error when accessing Visual area in 'statusline'

Problem:    ml_get error when accessing Visual area in 'statusline'.
Solution:   Disable Visual mode when using another window. (closes #5278)
diff --git a/src/testdir/test_statusline.vim b/src/testdir/test_statusline.vim
index e1830d0..af05be5 100644
--- a/src/testdir/test_statusline.vim
+++ b/src/testdir/test_statusline.vim
@@ -368,3 +368,25 @@
   set laststatus&
   set splitbelow&
 endfunc
+
+func Test_statusline_visual()
+  func CallWordcount()
+    call wordcount()
+  endfunc
+  new x1
+  setl statusline=count=%{CallWordcount()}
+  " buffer must not be empty
+  call setline(1, 'hello')
+
+  " window with more lines than x1
+  new x2
+  call setline(1, range(10))
+  $
+  " Visual mode in line below liast line in x1 should not give ml_get error
+  call feedkeys("\<C-V>", "xt")
+  redraw
+
+  delfunc CallWordcount
+  bwipe! x1
+  bwipe! x2
+endfunc