What I want is just the first section to stand until the second section completely covers it. Is there another way to achieve this?
Remove your code for addon fixed position, first section z-index and footer.
Set the fixed height for the first section.
Set margin-top in second section equal to the height of the first section.
Add class name to the first section something like first-screen
Use following code
.first-screen {
position: fixed !important;
top: 0;
}
#sp-footer {
position: relative;
z-index: 1;
}
Set padding-top in first section to сompensate Header height.
If required for mobile devices, change the height of the first section and margin-top in second section.