[scroll-animations-1] Copy over layout cycles section

Author: fantasai

scroll-animations-1/rewrite.bs

@@ -606,3 +606,49 @@ spec: cssom-view-1; type: dfn;
 	[=scroll progress timelines=] take precedence over [=view progress timelines=].
 
 	ISSUE: Do we want to expand this scope to preceding siblings and/or distant cousins?
+
+# Avoiding cycles with layout # {#avoiding-cycles}
+
+	The ability for scrolling to drive the progress of an animation,
+	gives rise to the possibility of <dfn>layout cycles</dfn>,
+	where a change to a scroll offset causes an animation's effect to update,
+	which in turn causes a new change to the scroll offset.
+
+	To avoid such [=layout cycles=],
+	animations with a [=scroll progress timeline=] are sampled once per frame,
+	after scrolling in response to input events has taken place,
+	but before {{requestAnimationFrame()}} callbacks are run.
+	If the sampling of such an animation causes a change to a scroll offset,
+	the animation will not be re-sampled to reflect the new offset
+	until the next frame.
+
+	The implication of this is that in some situations, in a given frame,
+	the rendered scroll offset of a scroll container might not be consistent
+	with the state of an animation driven by scrolling that scroll container.
+	However, this will only occur in situations where the animation's effect
+	changes the scroll offset of that same scroll container
+	(in other words, in situations where the animation's author is asking for trouble).
+	In normal situations, including&mdash;importantly--
+	when scrolling happens in response to input events,
+	the rendered scroll offset and the state of scroll-driven animations
+	will be consistent in each frame.
+
+	User agents that composite frames asynchronously
+	with respect to layout and/or script
+	may, at their discretion, sample scroll-driven animations
+	once per <em>composited</em> frame,
+	rather than (or in addition to) once per full layout cycle.
+	Again, if sampling such an animation causes a change to a scroll offset,
+	the animation will not be re-sampled to reflect the new offset
+	until the next frame.
+
+	Nothing in this section is intended to require
+	that scrolling block on layout or script.
+	If a user agent normally composites frames where scrolling has occurred
+	but the consequences of scrolling have not been fully propagated in layout or script
+	(for example, `scroll` event listeners have not yet run),
+	the user agent may likewise choose not to sample scroll-driven animations
+	for that composited frame.
+	In such cases, the rendered scroll offset
+	and the state of a scroll-driven animation
+	may be inconsistent in the composited frame.