I need some help understanding what is happening on scroll, but before any scroll event handlers are called. I have the following performance snapshot:

We can see in "Function call" that Total time is 1.48s, where self time is 0.12ms. There is no sub-tree of functions during this time. The actual event handlers are executed at the end of this period, around 1900ms. The "Aggregated Time" panel suggests that the "System" is taking up 1481ms of "Function call". But as I haven't made any scripted change to the DOM yet, what on earth is the system doing?

Have I interpreted the graph correctly? If not, what am I missing?