It's not irrelevant, it's right at the heart of the problem! Modern browsers use separate threads for Javascript and rendering. But if you have a (non-passive)

It's not irrelevant, it's right at the heart of the problem! Modern browsers use separate threads for Javascript and rendering. But if you have a (non-passive) scroll event, the rendering thread has to block on the Javascript thread, and that's why scrolling is janky.