fix(cmdheight=0): various issues part3 #19816

And fixed in https://github.com/neovim/neovim/pull/19801 regression.

Fix #19834
Fix #19184

src/nvim/getchar.c

@@ -1698,6 +1698,7 @@ static void getchar_common(typval_T *argvars, typval_T *rettv)
   if (!ui_has_messages()) {
     // redraw the screen after getchar()
     update_screen(UPD_NOT_VALID);
+    clear_cmdline = true;
   }
 
   set_vim_var_nr(VV_MOUSE_WIN, 0);