window.scrollTo/scrollBy/scroll were empty stubs, scrollX/scrollY and
pageXOffset/pageYOffset were hard-coded 0 data properties, no scroll
event was ever fired at the window, and document.scrollingElement did
not exist.
that "scroll-driven lazy loaders advance instead of throwing". But the
dominant infinite-scroll idiom is window-level:
window.scrollTo(0, document.body.scrollHeight);
if (window.scrollY + innerHeight >= body.scrollHeight) loadMore();
all of which was inert -- the call succeeded, the offset never moved, the
sentinel never tripped -- so automation that scrolls a page to trigger
loading silently got one screenful.
The offset is stored on the scrolling element rather than in separate
window state, so window.scrollY and document.scrollingElement.scrollTop
are two views of one value in both directions, which is what pages
assume when they mix the two. scrollX/scrollY/pageXOffset/pageYOffset
become read-only accessors over it, matching a real Window, and
document.scrollingElement returns documentElement (standards mode; we
never parse in quirks mode).
The scroll event is dispatched at both the document and the window.
Chrome propagates a page scroll to both, but Document.dispatchEvent here
runs only its own listeners and does not propagate, so firing once would
strand half the listeners.
As with #431 there is no layout, so the offset still cannot be clamped to
a real maximum; negative values clamp to 0 exactly as they do for
elements.
Andrey Mnatsakanov
f6ca133132