In the menu, only one section can be expanded at a time. If you have one section expanded, and then click on any other section header to expand the section below that one, the header you clicked on is no longer under your mouse but instead has immediately shifted up. It disrupts the flow, and I have a moment of confusion of where I just was or what I just clicked on.
I assume this was a decision made to prevent users from expanding every section and making the menu really long. If you're dead set on only allowing one section expanded at a time, then perhaps experiment with a transition/animation, so it's a little more obvious what's going on and I might not lose my place so easily.