added-active-view-transition (#12635)

Author: vmpstr

css-view-transitions-2/Overview.bs

@@ -506,7 +506,7 @@ This will activate any of the following selectors:
 :root:active-view-transition {}
 ```
 
-While starting a transition without providing transition types, would only activate ':active-view-transition'':
+While starting a transition without providing transition types, would only activate '':active-view-transition'':
 
 ```js
 document.startViewTransition(updateTheDOMSomehow);
@@ -755,6 +755,21 @@ div.box {
 		1. Return |viewTransition|.
 	</div>
 
+## Exposing current transition: `activeViewTransition` ## {#active-view-transition}
+
+In order to provide ergonomic behavior, [=active view transition=] is exposed to script via a document property.
+
+<dl dfn-for=document>
+	: <dfn>activeViewTransition</dfn>
+	:: a {{ViewTransition}} object, or null,
+		that represents [=active view transition=].
+</dl>
+
+Note: [=activeViewTransition=] mirrors [=active view transition=] in that whenever the latter exists
+(there is an ongoing transition), then [=activeViewTransition=] is not null and refers to this object
+
+ISSUE: When merging css-view-transitions-1 into this, we can rename [=active view transition=] to [=activeViewTransition=]
+
 # Determining 'view-transition-name' automatically # {#auto-vt-name}
 
 ## Overview ## {#auto-vt-name-overview}
@@ -1340,7 +1355,7 @@ this still presents two potential threats:
 
 1. The [=environment settings object/cross-origin isolated capability=] in both documents might be different.
 	This can cause a situation where a {{Document}} that is [=environment settings object/cross-origin isolated capability|cross-origin isolated=]
-	can read image data from a document that is not cross-origin isolated. This is already mitigated in [[css-view-transitions-1#sec],
+	can read image data from a document that is not cross-origin isolated. This is already mitigated in [[css-view-transitions-1#sec]],
 	as the same restriction applies for captured cross-origin iframes.
 
 1. A same-origin navigation might still occur via a cross-origin redirect, e.g. <code>https://example.com</code>
@@ -1383,3 +1398,4 @@ Changes from <a href="https://www.w3.org/TR/2024/WD-css-view-transitions-2-20240
 * Initial spec for nested view transitions (<a href="https://github.com/w3c/csswg-drafts/issues/10334">#10334</a>)
 * `view-transition-class` is a tree-scoped name (<a href="https://github.com/w3c/csswg-drafts/issues/10529">#10529</a>)
 * Stop extending CSSRule with obsolete pattern (See <a href="https://github.com/w3c/csswg-drafts/issues/10606">#10606</a>)
+* Added [=activeViewTransition=] definition (<a href="https://github.com/w3c/csswg-drafts/issues/12407">#12407</a>)