It didn't (fully), but adding it wouldn't have been more than a days work. You have a hierarchy of enclosing layout rectangles, you take an invalidation rectangle, and you recursively intersect children and them draw them with the painters algorithm.

Vestiges of the system did exist, as there were scrollable panels and controls had an idea that they were not visible due to falling outside of their parent rect, and all drawing functions were aware of being clamped to a boundary box.

So I'd say some of the hard(ish) stuff was already done, and people did end up using it in production so it actual survived contact with a paying customer, which is saying something.

Partial redraws are mostly a waste of complexity at this point. It's much faster easier just to re-layout and re-draw the whole thing in a frame. It can be done in a millisecond if you're not beholden to the browser.