Why dropdown has no border when disabled?

How-to
#1

lv_obj_add_state(t_dd, LV_STATE_DISABLED);

When button 、textarea in disabled state,there is a border,but the dropdown does not.

What should I do?

How to disable a drop-down list?
#2

@embeddedt Do you know how to solve it?

#3

With the built-in material theme there is a border for me:
image