Thanks for this infomation about using sprite object to replace box object to make a boundary.
Another question, where to use box object?
In your cap, when black sprite idle(no keyboard pressed), it
had not been push when ball sprite hit it. It looks like a wall to block the ball. How to do that?
In my cap, the black sprite will be push back until ouside the window. I see the position x of black sprite is less then 0 in debug mode.
If black sprite will not move whe ball hit it, it will not been push ouside the window. That is a good solution.
1, You could still use the box objects for the walls and floor, but you need to fill it with a colour (untick its 'use' transparent attribute for the fill colour)
2, To stop the black sprite moving when the ball collides : Give the black sprite a 'solid' attribute (tick the box that makes it solid)