[css-anchor-position] Record last-successful position option at the same time as we forget. Specify more precisely *what* is being remembered. #10216

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1809,8 +1809,7 @@ even tho they depend on layout and [=used values=].
 		4. If |el rect| is not fully contained within |cb rect|,
 			[=iteration/continue=].
 
-		5. Set |option| as |el|'s <dfn>last successful position option</dfn>.
-			Return |adjusted styles|.
+		5. Return |adjusted styles|.
 
 	3. Assert: The previous step finished without finding a [=position option=]
 		that avoids overflow.
@@ -1846,18 +1845,29 @@ laying out later elements cannot change this decision.
 	Layout does not "go backward", in other words.
 </div>
 
-<div algorithm="forget the last rememberd @position-try option">
-	At the time that {{ResizeObserver}} events are determined and delivered,
-	if an element |el| has a [=last successful position option=]
-	and if any of the following are true of it,
-	remove its [=last successful position option=]:
+<div algorithm="remember or forget the last rememberd position option">
+	At the time that {{ResizeObserver}} events are determined and delivered:
 
-	* |el| is not an [=absolutely positioned=] element
-	* |el|'s computed value for 'position-try-options' has changed
-	* Any of the ''@position-try'' rules referenced by |el|'s 'position-try-options'
-		have been added, removed, or mutated.
+	* If an element |el| is [=absolutely positioned=],
+		set its [=last successful position option=]
+		to the set of [=accepted @position-try properties=] (and values)
+		that it's currently using.
 
-	Note: The timing of this removal
+	* Otherwise, if |el| has a [=last successful position option=]
+		and if any of the following are true of it,
+		remove its [=last successful position option=]:
+
+		* |el| is not an [=absolutely positioned=] element
+		* |el|'s computed value for 'position-try-options' has changed
+		* Any of the ''@position-try'' rules referenced by |el|'s 'position-try-options'
+			have been added, removed, or mutated.
+
+		Then, [=determine the position fallback styles=] of |el|
+		and set its [=last successful position option=]
+		to the set of [=accepted @position-try properties=] (and values)
+		that it's now using.
+
+	Note: The timing of this recording/removal
 	is intentionally identical to the treatment of [=last remembered sizes=].
 </div>