[css-anchor-position] Add more conditions for forgetting the last remembered @position-try option. #10099

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1561,11 +1561,20 @@ laying out later elements cannot change this decision.
 	Layout does not "go backward", in other words.
 </div>
 
-At the time that {{ResizeObserver}} events are determined and delivered,
-if an element has a [=last successful position option=]
-but is not an [=absolutely positioned=] element,
-remove its [=last successful position option=].
-(This is intentionally identical to the treatment of [=last remembered sizes=].)
+<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=]:
+
+	* |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.
+
+	Note: The timing of this removal
+	is intentionally identical to the treatment of [=last remembered sizes=].
+</div>
 
 Issue: Make sure the snapshotted scroll offset stuff is correct now,
 given interleaving.