An observation...
On the desktop, the sidebar is locked/aligned to the bottom of the content area. In other words, when you start scrolling down the page, the main content area scrolls down, but the sidebar remains static. As you continue to scroll and get closer to the bottom of the page, the sidebar will start to scroll and will then stop scrolling at the same time as the main content area.
Alternatively, if the sidebar were aligned with the top of the main content area, it would scroll at the same time as the main content area (and would stop scrolling before the main content area stops scrolling, if the content in the sidebar were less lengthy than the content in the main content area).