[css-ruby] Clarify white space collapsing

Author: frivoal

css-ruby-1/Overview.bs

@@ -645,12 +645,20 @@ White Space Collapsing</h3>
 	</div>
 
 	Where undiscarded white space is [=collapsible white space|collapsible=],
-	it will collapse across consecutive elements
+	it will collapse across adjacent boxes in each line box
 	following the standard <a href="https://www.w3.org/TR/css3-text/#white-space-rules">white space processing rules</a> [[!CSS-TEXT-3]].
-	For [=collapsible white space=] between [=ruby segments=] ([=inter-segment white space=]), however,
+	Annotations in separate [=ruby segments=]
+	or separated by [=auto-hidden annotations=]
+	are not considered adjacent;
+	however all base-level content
+	(including [=inter-character annotation boxes=],
+	which are treated as [=atomic inlines=])
+	is.
+	For [=collapsible white space=] between [=ruby segments=] ([=inter-segment white space=]),
 	the contextual text for determining [[css-text-3#line-break-transform|segment break transformations]]
-	is given by the [=ruby bases=] on either side,
-	not the text on either side of the white space in source document order.
+	is thus given by the [=ruby bases=] on either side,
+	not necessarily the text on either side of the white space in source document order
+	(which could include [=interlinear annotations=]).
 
 	<div class="note">
 		Note: The white space processing rules