pushing past the canvas and onto the frame’s border
It’s worded a bit odd, but it is pushing against the padding on .frame.
It’s margin collapse. You can think of it as the child’s margin “overflowing” its parent container. Try removing the padding on .frame, the white space you see is the top margin on .one pushing outside the parent .canvas container and pushing against the border on .frame.
If you continue with the next challenge it shows how to establish a new Block Formatting Context by using overflow: hidden.