fix(getchar): flush screen before doing a blocking wait

src/nvim/eval/funcs.c

@@ -2722,6 +2722,8 @@ static void getchar_common(typval_T *argvars, typval_T *rettv)
       // getchar(): blocking wait.
       // TODO(bfredl): deduplicate shared logic with state_enter ?
       if (!char_avail()) {
+        // flush output before waiting
+        ui_flush();
         (void)os_inchar(NULL, 0, -1, 0, main_loop.events);
         if (!multiqueue_empty(main_loop.events)) {
           state_handle_k_event();