[scroll-animations-1] Use logical coords for scroll offsets. #7752

scroll-animations-1/Overview.bs

@@ -555,23 +555,22 @@ spec: cssom-view-1; type: dfn;
  : <dfn>startOffset</dfn>
  :: Represents the starting (0% progress) scroll position
  of the [=view progress timeline=]
- as a length offset (in ''px'') from the scroll origin.
+ as a length offset (in ''px'') from the [=scroll origin=].
  Null when the timeline is [=inactive timeline|inactive=].
 
  : <dfn>endOffset</dfn>
  :: Represents the ending (100% progress) scroll position
  of the [=view progress timeline=]
- as a length offset (in ''px'') from the scroll origin.
+ as a length offset (in ''px'') from the [=scroll origin=].
  Null when the timeline is [=inactive timeline|inactive=].
  </dl>
 
- Note: The value of {{ViewTimeline/startOffset}} can be greater than {{ViewTimeline/endOffset}},
+ Note: The values of {{ViewTimeline/startOffset}} and {{ViewTimeline/endOffset}}
+ are relative to the [=scroll origin=], not the [=physical=] top left corner.
+ Depending on the [=writing mode=] of the [=scroll container=],
+ they therefore might not match {{Element/scrollLeft}} or {{Element/scrollTop}} values,
  for example in the [=horizontal axis=] in a right-to-left (''rtl'') [=writing mode=].
 
- ISSUE: This makes the offsets compatible with the way scrollLeft and scrollTop work,
- which go negative from zero in RTL scroll containers,
- is this how we want it to work here?
-
  Inherited attributes:
 
  <dl>