'titleold': simplify behavior

- default 'titleold' to empty
- set title on exit if 'title' is enabled and 'titleold' is non-empty
- update docs

src/nvim/buffer.c

@@ -3069,14 +3069,11 @@ static bool ti_change(char_u *str, char_u **last)
 /// Set current window title
 void resettitle(void)
 {
-  // if icon change, should the title be reset too?
   if (p_icon) {
-    ui_call_set_title(cstr_as_string((char *)lasttitle));
     ui_call_set_icon(cstr_as_string((char *)lasticon));
-  } else if (p_title) {
-    ui_call_set_title(cstr_as_string((char *)lasttitle));
   }
   if (p_title || p_icon) {
+    ui_call_set_title(cstr_as_string((char *)lasttitle));
     ui_flush();
   }
 }

src/nvim/globals.h

@@ -279,7 +279,6 @@ EXTERN int need_wait_return INIT(= 0);      /* need to wait for return later */
 EXTERN int did_wait_return INIT(= FALSE);       /* wait_return() was used and
                                                    nothing written since then */
 EXTERN int need_maketitle INIT(= TRUE);      /* call maketitle() soon */
-EXTERN int did_enable_title INIT(= FALSE);   /* did set title */
 
 EXTERN int quit_more INIT(= FALSE);         /* 'q' hit at "--more--" msg */
 #if defined(UNIX) || defined(MACOS_X)

src/nvim/option.c

@@ -1953,7 +1953,6 @@ did_set_title (
   if (starting != NO_SCREEN) {
     maketitle();
     resettitle();
-    did_enable_title = true;
   }
 }
 

src/nvim/options.lua

@@ -2498,7 +2498,7 @@ return {
       no_mkrc=true,
       vi_def=true,
       varname='p_titleold',
-      defaults={if_true={vi=N_("Thanks for flying Vim")}}
+      defaults={if_true={vi=N_("")}}
     },
     {
       full_name='titlestring',

src/nvim/os_unix.c

@@ -138,10 +138,7 @@ void mch_exit(int r) FUNC_ATTR_NORETURN
 {
   exiting = true;
 
-  if ((p_title
-       || (did_enable_title
-           && (p_titlestring == NULL || STRLEN(p_titlestring) == 0)))
-      && p_titleold != NULL) {
+  if (p_title && *p_titleold != NUL) {
     ui_call_set_title(cstr_as_string((char *)p_titleold));
   }
   ui_builtin_stop();