Hi, I have viewed your codepen and your issue stems from your #welcome {} selector, in which you have placed a non-specific padding around the element.
This means in addition to pushing it downwards, avoiding it being blocked by your nav-bar with the position:fixed rule, your padding also adds extra width/height to the side of the welcome section and bottom.
This is also the reason why you have a horizontal scrollbar in addition to the vertical.
To fix, either delete the padding property and instead use:
position:relative;
top:5.25rem;