Mark shouldStartViewTransition as true when there's an enter animation

packages/react-reconciler/src/ReactFiberCommitViewTransitions.js

@@ -67,6 +67,20 @@ export function trackAppearingViewTransition(
   appearingViewTransitions.set(name, state);
 }
 
+export function trackEnterViewTransitions(placement: Fiber): void {
+  if (
+    placement.tag === ViewTransitionComponent ||
+    (placement.subtreeFlags & ViewTransitionStatic) !== NoFlags
+  ) {
+    // If an inserted or appearing Fiber is a ViewTransition component or has one as
+    // an immediate child, then that will trigger as an "Enter" in future passes.
+    // We don't do anything else for that case in the "before mutation" phase but we
+    // still have to mark it as needing to call startViewTransition if nothing else
+    // updates.
+    shouldStartViewTransition = true;
+  }
+}
+
 // We can't cancel view transition children until we know that their parent also
 // don't need to transition.
 export let viewTransitionCancelableChildren: null | Array<
@@ -119,7 +133,6 @@ function applyViewTransitionToHostInstancesRecursive(
   let inViewport = false;
   while (child !== null) {
     if (child.tag === HostComponent) {
-      shouldStartViewTransition = true;
       const instance: Instance = child.stateNode;
       if (collectMeasurements !== null) {
         const measurement = measureInstance(instance);
@@ -132,6 +145,7 @@ function applyViewTransitionToHostInstancesRecursive(
           inViewport = true;
         }
       }
+      shouldStartViewTransition = true;
       applyViewTransitionName(
         instance,
         viewTransitionHostInstanceIdx === 0

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -235,6 +235,7 @@ import {
   commitFragmentInstanceInsertionEffects,
 } from './ReactFiberCommitHostEffects';
 import {
+  trackEnterViewTransitions,
   commitEnterViewTransitions,
   commitExitViewTransitions,
   commitBeforeUpdateViewTransition,
@@ -338,6 +339,9 @@ function commitBeforeMutationEffects_begin(isViewTransitionEligible: boolean) {
       // to trigger updates of any nested view transitions and we shouldn't
       // have any other before mutation effects since snapshot effects are
       // only applied to updates. TODO: Model this using only flags.
+      if (isViewTransitionEligible) {
+        trackEnterViewTransitions(fiber);
+      }
       commitBeforeMutationEffects_complete(isViewTransitionEligible);
       continue;
     }
@@ -367,6 +371,9 @@ function commitBeforeMutationEffects_begin(isViewTransitionEligible: boolean) {
           // to trigger updates of any nested view transitions and we shouldn't
           // have any other before mutation effects since snapshot effects are
           // only applied to updates. TODO: Model this using only flags.
+          if (isViewTransitionEligible) {
+            trackEnterViewTransitions(fiber);
+          }
           commitBeforeMutationEffects_complete(isViewTransitionEligible);
           continue;
         }