How to make a button in the top of the screen to be displayed at all times and a button in the bottom of the screen to be visible only when scrolled down?

How to make a button in the top of the screen to be displayed at all times and a button in the bottom of the screen to be visible only when scrolled down ? I have disabled scroll in the screen1 and used a vertical scroll arrengment layout as the screen displays more than full screen amount of text content