vim-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 vim/vim#5278)
dee50a5180

src/nvim/buffer.c

@@ -3801,14 +3801,20 @@ int build_stl_str_hl(
 
       buf_T *const save_curbuf = curbuf;
       win_T *const save_curwin = curwin;
+      const int save_VIsual_active = VIsual_active;
       curwin = wp;
       curbuf = wp->w_buffer;
+      // Visual mode is only valid in the current window.
+      if (curwin != save_curwin) {
+        VIsual_active = false;
+      }
 
       // Note: The result stored in `t` is unused.
       str = eval_to_string_safe(out_p, &t, use_sandbox);
 
       curwin = save_curwin;
       curbuf = save_curbuf;
+      VIsual_active = save_VIsual_active;
 
       // Remove the variable we just stored
       do_unlet(S_LEN("g:actual_curbuf"), true);

src/nvim/testdir/test_statusline.vim

@@ -347,3 +347,25 @@ func Test_statusline()
   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