Fix measurement bug

We were reading from the wrong fiber.

Update the fixture to show that the document and the button remains interactive
due to the inner boundary not resizing.

fixtures/view-transition/src/components/Page.js

@@ -46,30 +46,34 @@ export default function Page() {
         }}>
         {show ? 'A' : 'B'}
       </button>
-      {show ? (
-        <div>
-          {a}
-          {b}
-        </div>
-      ) : (
+      <ViewTransition>
         <div>
-          {b}
-          {a}
+          {show ? (
+            <div>
+              {a}
+              {b}
+            </div>
+          ) : (
+            <div>
+              {b}
+              {a}
+            </div>
+          )}
+          <ViewTransition>
+            {show ? <div>hello{exclamation}</div> : <section>Loading</section>}
+          </ViewTransition>
+          {show ? null : (
+            <ViewTransition>
+              <div>world{exclamation}</div>
+            </ViewTransition>
+          )}
+          <Activity mode={show ? 'visible' : 'hidden'}>
+            <ViewTransition>
+              <div>!!</div>
+            </ViewTransition>
+          </Activity>
         </div>
-      )}
-      <ViewTransition>
-        {show ? <div>hello{exclamation}</div> : <section>Loading</section>}
       </ViewTransition>
-      {show ? null : (
-        <ViewTransition>
-          <div>world{exclamation}</div>
-        </ViewTransition>
-      )}
-      <Activity mode={show ? 'visible' : 'hidden'}>
-        <ViewTransition>
-          <div>!!</div>
-        </ViewTransition>
-      </Activity>
     </div>
   );
 }

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -823,7 +823,7 @@ function commitExitViewTransitions(
   }
 }
 
-function commitBeforeUpdateViewTransition(fiber: Fiber): void {
+function commitBeforeUpdateViewTransition(current: Fiber): void {
   // The way we deal with multiple HostInstances as children of a View Transition in an
   // update can get tricky. The important bit is that if you swap out n HostInstances
   // from n HostInstances then they match up in order. Similarly, if you don't swap
@@ -840,12 +840,12 @@ function commitBeforeUpdateViewTransition(fiber: Fiber): void {
   // be unexpected but it is in line with the semantics that the ViewTransition is its
   // own layer that cross-fades its content when it updates. If you want to reorder then
   // each child needs its own ViewTransition.
-  const name = getViewTransitionName(fiber.memoizedProps, fiber.stateNode);
+  const name = getViewTransitionName(current.memoizedProps, current.stateNode);
   viewTransitionHostInstanceIdx = 0;
   applyViewTransitionToHostInstances(
-    fiber.child,
+    current.child,
     name,
-    (fiber.memoizedState = []),
+    (current.memoizedState = []),
     true,
   );
 }
@@ -1074,7 +1074,7 @@ function measureUpdateViewTransition(
   // If nothing changed due to a mutation, or children changing size
   // and the measurements end up unchanged, we should restore it to not animate.
   viewTransitionHostInstanceIdx = 0;
-  const previousMeasurements = finishedWork.memoizedState;
+  const previousMeasurements = current.memoizedState;
   const inViewport = measureViewTransitionHostInstances(
     current,
     finishedWork,