Surprise: scrollbars are complicated, especially cross-browser and cross-platform.
Sometimes they take up space and sometimes they don’t. Sometimes that is affected by a setting and sometimes it isn’t. Sometimes you can see them and sometimes you can’t unless you’re actually scrolling. Styling them is handled in all sorts of different ways, including some very recent developments.
Follow Zach’s journey toward thinner, native, user-preference-respecting, more aesthetic scrollbars, particularly for element-level scrollbars that ends up here.