patch 8.1.0865: when 'listchars' only contains "nbsp:X" it does not work

Problem:    When 'listchars' only contains "nbsp:X" it does not work.
Solution:   Set extra_check when lcs_nbsp is set. (Ralf Schandl, closes #3889)

src/screen.c

@@ -1164,8 +1164,6 @@ win_update(win_T *wp)
 	    mod_bot = wp->w_redraw_bot + 1;
 	else
 	    mod_bot = 0;
-	wp->w_redraw_top = 0;	/* reset for next time */
-	wp->w_redraw_bot = 0;
 	if (buf->b_mod_set)
 	{
 	    if (mod_top == 0 || mod_top > buf->b_mod_top)
@@ -1277,6 +1275,8 @@ win_update(win_T *wp)
 	if (mod_top != 0 && buf->b_mod_xlines != 0 && wp->w_p_nu)
 	    mod_bot = MAXLNUM;
     }
+    wp->w_redraw_top = 0;	// reset for next time
+    wp->w_redraw_bot = 0;
 
     /*
      * When only displaying the lines at the top, set top_end.  Used when
@@ -3498,7 +3498,7 @@ win_line(
 
     if (wp->w_p_list)
     {
-	if (lcs_space || lcs_trail)
+	if (lcs_space || lcs_trail || lcs_nbsp)
 	    extra_check = TRUE;
 	/* find start of trailing whitespace */
 	if (lcs_trail)

src/testdir/test_listchars.vim

@@ -90,6 +90,26 @@ func Test_listchars()
 	      \ '.....h>-$',
 	      \ 'iii<<<<><<$', '$'], l)
 
+
+  " test nbsp
+  normal ggdG
+  set listchars=nbsp:X,trail:Y
+  set list
+  " Non-breaking space
+  let nbsp = nr2char(0xa0)
+  call append(0, [ ">".nbsp."<" ])
+
+  let expected = '>X< '
+
+  redraw!
+  call cursor(1, 1)
+  call assert_equal([expected], ScreenLines(1, virtcol('$')))
+
+  set listchars=nbsp:X
+  redraw!
+  call cursor(1, 1)
+  call assert_equal([expected], ScreenLines(1, virtcol('$')))
+
   enew!
   set listchars& ff&
 endfunc

src/version.c

@@ -783,6 +783,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    865,
 /**/
     864,
 /**/