[css-anchor-position-1] Clarify interaction with transitions and animations (tentative) w3c#13048

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -1993,6 +1993,9 @@ and thus trigger special behavior. These <dfn>fallback-sensitive changes</dfn> i
 	* Any of the ''@position-try'' rules referenced by it
 		have been added, removed, or mutated.
 
+<h5 id="last-successful-recording">
+Recording the [=last successful position option=]</h5>
+
 In order to maintain layout stability as much as possible,
 [=determining position fallback styles=] prioritizes the [=last successful position option=],
 which is determined as follows:
@@ -2018,6 +2021,34 @@ which is determined as follows:
 	is intentionally identical to the treatment of [=last remembered sizes=].
 </div>
 
+ISSUE(13048): The following sections attempt to clarify the interaction with transitions and animations.
+
+<h5 id="suspending-fallback-transitions">
+Suspending Fallback During Transitions</h5>
+
+The UA must [=determine position fallback styles=]
+for both the start and end states of a [=transition=]
+(see [[CSS-TRANSITIONS-1]])
+that includes properties that could cause a [=fallback-sensitive change=].
+
+During a [=transition=] for properties that could cause a [=fallback-sensitive change=], however,
+[=determining position fallback styles=] and
+[=recording the last successful position option=] are suspended.
+
+<h5 id="suspending-fallback-animations">
+Suspending Fallback During Animations</h5>
+
+If an [=animation=] (see [[CSS-ANIMATIONS-1]] and [[WEB-ANIMATIONS-1]])
+affects any properties that could cause a [=fallback-sensitive change=],
+then the UA must [=determine position fallback styles=]
+for the keyframes that contain those properties (only).
+As fallback determination is order-sensitive,
+later keyframes must take into account the result of earlier keyframes.
+
+While animating between these keyframes, however,
+[=determining position fallback styles=] and
+[=recording the last successful position option=] are suspended.
+
 <h4 id="applying-options">
 Applying Position Options</h4>
 
@@ -2652,6 +2683,11 @@ No Privacy issues have been raised against this document.
 Changes {#changes}
 =======
 
+Significant changes since the <a href="https://www.w3.org/TR/2025/WD-css-anchor-position-1-20251222/">22 December 2025 Working Draft</a>:
+
+* Clarify positoin fallback interaction with transitions and animation.
+	(<a href="https://github.com/w3c/csswg-drafts/issues/13048">Issue 13048</a>)
+
 Significant changes since the <a href="https://www.w3.org/TR/2025/WD-css-anchor-position-1-20251007/">7 October 2025 Working Draft</a>:
 
 * Add ''flip-x'' and ''flip-y'' to 'position-try-fallbacks'.