[css-position-4] Switch 'adding to the top layer when it's already in it' to actually be a spec error, as removals/reshuffles should be handled properly.

Author: tabatkins

css-position-4/Overview.bs

@@ -455,11 +455,13 @@ Top Layer Manipulation {#top-manip}
 	given a {{Document}} |doc|
 	and an {{Element}} |el|:
 
+	1. If |el| is already [=list/contained=] in |doc|'s [=Document/top layer=]:
+		* Assert: |el| is also in |doc|'s [=pending top layer removals=].
+			(Otherwise, this is a spec error.)
+		* [=set/Remove=] |el| from both |doc|'s [=Document/top layer=]
+			and [=pending top layer removals=].
 
-	1. [=set/Remove=] |el| from |doc|'s [=Document/top layer=],
-		then [=set/append=] |el| to |doc|'s [=Document/top layer=].
-
-	2. [=set/Remove=] |el| from |doc|'s [=pending top layer removals=].
+	2. [=set/Append=] |el| to |doc|'s [=Document/top layer=].
 
 	3. At the UA !important [=cascade origin=],
 		add a rule targeting |el|
@@ -512,6 +514,10 @@ Top Layer Manipulation {#top-manip}
 	Note: This is intended to be called during the "Update Rendering" step
 	of HTML's rendering algorithm.
 	It is not intended to be called by other algorithms.
+
+	Note: The 'overlay' check can be delayed arbitrarily long
+	by author-level transitions;
+	see [[#overlay]] for details.
 </div>
 
 Controlling the Top Layer: the 'overlay' property {#overlay}