cmdheight=0: fix bugs part2 (#19185)

src/nvim/ui.c

@@ -612,6 +612,12 @@ bool ui_has(UIExtension ext)
   return ui_ext[ext];
 }
 
+/// Returns true if the UI has messages area.
+bool ui_has_messages(void)
+{
+  return p_ch > 0 || ui_has(kUIMessages);
+}
+
 Array ui_array(void)
 {
   Array all_uis = ARRAY_DICT_INIT;