feat(api): allow win_close in cmdwin to close wins except previous

Disallow closing the previous window from `nvim_win_close`, as this will cause
issues.

Again, no telling how safe this is. It also requires exposing old_curwin. :/

Also note that it's possible for the `&cmdheight` to change if, for example,
there are 2 tabpages and `nvim_win_close` is used to close the last window in
the other tabpage while `&stal` is 1. This is addressed in a later commit.

src/nvim/api/window.c

@@ -404,10 +404,11 @@ void nvim_win_close(Window window, Boolean force, Error *err)
   if (cmdwin_type != 0) {
     if (win == curwin) {
       cmdwin_result = Ctrl_C;
-    } else {
+      return;
+    } else if (win == cmdwin_old_curwin) {
       api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
+      return;
     }
-    return;
   }
 
   tabpage_T *tabpage = win_find_tabpage(win);

src/nvim/ex_getln.c

@@ -4398,6 +4398,7 @@ static int open_cmdwin(void)
   // Set "cmdwin_type" before any autocommands may mess things up.
   cmdwin_type = get_cmdline_type();
   cmdwin_level = ccline.level;
+  cmdwin_old_curwin = old_curwin;
 
   // Create empty command-line buffer.
   if (buf_open_scratch(0, _("[Command Line]")) == FAIL) {
@@ -4405,6 +4406,7 @@ static int open_cmdwin(void)
     win_close(curwin, true, false);
     ga_clear(&winsizes);
     cmdwin_type = 0;
+    cmdwin_old_curwin = NULL;
     return Ctrl_C;
   }
   // Command-line buffer has bufhidden=wipe, unlike a true "scratch" buffer.
@@ -4501,6 +4503,7 @@ static int open_cmdwin(void)
 
   cmdwin_type = 0;
   cmdwin_level = 0;
+  cmdwin_old_curwin = NULL;
 
   exmode_active = save_exmode;
 

src/nvim/globals.h

@@ -831,6 +831,7 @@ EXTERN bool km_startsel INIT(= false);
 EXTERN int cmdwin_type INIT(= 0);    ///< type of cmdline window or 0
 EXTERN int cmdwin_result INIT(= 0);  ///< result of cmdline window or 0
 EXTERN int cmdwin_level INIT(= 0);   ///< cmdline recursion level
+EXTERN win_T *cmdwin_old_curwin INIT(= NULL);  ///< curwin before opening cmdline window or NULL
 
 EXTERN char no_lines_msg[] INIT(= N_("--No lines in buffer--"));