I wonder how google indexes a page that inserts an element in the DOM 120 seconds after the page was loaded using a setTimeout()
My first guess would be that they snapshot the DOM in the JS tick immediately after window.onload completes. Maybe they have a short pause to let any fast timeouts or callbacks complete, but there's got to be a cutoff at some point (e.g. to stop an infinite wait for pages that continuously update a relative date). Of course, with their own JS engine, I bet they can get really fancy with the heuristics to determine when to take that snapshot.
If they're smart, they actually make the exact timeout a function of a HMAC of the loaded source, to make it very difficult to experiment around, find the exact limits, and fool the indexing system. Back in 2010, it was still a fixed time limit.
Source: executing JavaScript in Google's indexing pipeline was my job from 2006 to 2010.
Think of a long page where you need to read a few things as you scroll. You could game Google by timing it so that most humans would see content x but any script that ran at an unnatural speed would see y.
Imagine you use setInterval to load a new paragraph from a server, and the server only provides a new parapgraph 1200ms after the first polling?
Source: I primarily did JavaScript execution for Google's indexing pipeline 2006 to 2010.