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

Author: fantasai

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>