thanks @chasenlehara the problem is that I’m trying to use an external library and the library easily adds a class when you scroll the browser’s screen, so it looked simpler. I had other issues though that made me impossible to try other options but that’s another story.
So I guess the answer is to bind the library to the code
but that would take a lot of time in this case because of the other bug
so if a certain say div element gets a new class it will trigger a change in an observable in canjs