Sidebar pops to the top too soon while scrolling down

[brittag]

When I scroll down a little bit on the guide, I soon get to a point where the left sidebar "jumps" up and covers some of the introductory text (which I may still be reading at that point):

I would expect the left sidebar to not jump out of the normal page flow until I scroll all the way down to "Defining goals", where it makes sense to me that the sidebar starts floating.

This happens using Chrome and Safari on Mac.