Namronic

You have not yet scrolled to the part with the buttons so the part holding the two buttons is not yet in view of the screen (and this part must assume that the divs holding the two buttons have no position fixed property)

The part with the buttons are in view of the screen so they are somewhere on the screen, either top, left, bottom or right but all in all, you can see them somewhere

You have scrolled to the level of the buttons (where they would be if there was no position of fixed on their parents), so they are not out of view of the screen and can still be seen, the buttons are still visible whether partially or wholly (but this scenario assumes that there is no position fixed on the parents of the button and must still work even if it is there)

You have scrolled past the level of the buttons (where they would be if there was no position of fixed on their parents), so they are not in your view, this being out of view of the screen completely scenario/ case assumes that there is no position fixed on the parents of the button but it must work even if it is there and simply just assume it is not