there is a solution for that, just use a label with full width without any text and make the background color of your choice and turn off the has margin option from the advanced menu.
use a vertical arrangement to make this so the textbox will be in top and label in bottom. i think this is a alternative method for adding a line below the textbox
and one more thing, the textbox line is visible by default in the app, just update the makeroid companion app and then test it.
