Defer animation start time in FrameAnimationDriver (#56929)

Summary:

## Changelog:

[Internal] [Fixed] - Defer animation start time in FrameAnimationDriver

**Problem**: In complex apps, if animation is started in commit phase (the case if animation starts in useLayoutEffect, or from ViewTransition event handlers), it'll skip initial frames — the user sees the animation snap to an intermediate position. This happens because `FrameAnimationDriver` anchors its start time on the first `runAnimationStep` call, but the UI thread may be busy with layout/mount work for several frames before the view actually composites. The elapsed wall-clock time advances, causing `frameIndex` to jump ahead.

**Why**: `startFrameTimeMs_` is set to the Choreographer frame time on the first tick. If the UI thread is blocked processing a heavy tree (many views mounting), subsequent ticks arrive much later — `timeDeltaMs` jumps and the animation skips to a mid-point. 
- Every major framework solves this: Flutter uses lazy start (`_startTime ??= timeStamp` on first actual tick), Android native uses `CALLBACK_COMMIT` to adjust post-traversal, and CSS View Transitions spec defers start until post-composite.

**Fix**: On the very first `update()` call, output the starting value (frame 0) and reset `startFrameTimeMs_ = -1`. This causes the base class to re-anchor on the next `runAnimationStep`, so elapsed time is measured from the first frame that has actually been rendered — not from when `startAnimatingNode` was dispatched. The flag disables itself after one use, so all subsequent frames use pure elapsed-time with no behavioral change.

Differential Revision: D106007152

Author: zeyap

packages/react-native/ReactCommon/react/renderer/animated/drivers/FrameAnimationDriver.cpp

@@ -49,15 +49,34 @@ void FrameAnimationDriver::onConfigChanged() {
     frames_.push_back(frameValue);
   }
   toValue_ = config_["toValue"].asDouble();
+  auto deferIt = config_.find("deferredStart");
+  deferredStart_ = deferIt != config_.items().end() && deferIt->second.asBool();
 }
 
-bool FrameAnimationDriver::update(double timeDeltaMs, bool /*restarting*/) {
+bool FrameAnimationDriver::update(double timeDeltaMs, bool restarting) {
   if (auto node =
           manager_->getAnimatedNode<ValueAnimatedNode>(animatedValueTag_)) {
     if (!startValue_) {
       startValue_ = node->getRawValue();
     }
 
+    if (deferredStart_ && restarting) {
+      // On the very first update after start: output the starting value
+      // (frame 0) and defer the time anchor. The base class will re-anchor
+      // startFrameTimeMs_ on the next call, so elapsed time is measured
+      // from the first frame that has actually been rendered — not from
+      // when startAnimatingNode was dispatched.
+      //
+      // This prevents skipping initial frames when the UI thread is busy
+      // with layout/mount work between animation start and first composite.
+      node->setRawValue(
+          startValue_.value() + frames_[0] * (toValue_ - startValue_.value()));
+      markNodeUpdated(node->tag());
+      startFrameTimeMs_ = -1;
+      deferredStart_ = false;
+      return false;
+    }
+
     const auto startIndex =
         static_cast<size_t>(std::round(timeDeltaMs / SingleFrameIntervalMs));
     assert(startIndex >= 0);

packages/react-native/ReactCommon/react/renderer/animated/drivers/FrameAnimationDriver.h

@@ -35,6 +35,7 @@ class FrameAnimationDriver : public AnimationDriver {
   std::vector<double> frames_{};
   double toValue_{0};
   std::optional<double> startValue_{};
+  bool deferredStart_{false};
 };
 
 } // namespace facebook::react

packages/react-native/ReactCommon/react/renderer/animated/tests/AnimationDriverTests.cpp

@@ -117,4 +117,52 @@ TEST_F(AnimationDriverTests, framesAnimationReconfigurationClearsFrames) {
   EXPECT_EQ(round(nodesManager_->getValue(valueNodeTag).value()), toValue2);
 }
 
+TEST_F(AnimationDriverTests, framesAnimationDeferredStart) {
+  // Deferred start outputs frame 0 on the first update and re-anchors
+  // startFrameTimeMs_ so the second update also sees timeDelta=0.
+  // Without the defer the second frame would already be at value 25.
+  initNodesManager();
+
+  auto rootTag = getNextRootViewTag();
+
+  auto valueNodeTag = ++rootTag;
+  nodesManager_->createAnimatedNode(
+      valueNodeTag,
+      folly::dynamic::object("type", "value")("value", 0)("offset", 0));
+
+  const auto animationId = 1;
+  const auto frames = folly::dynamic::array(0.0f, 0.25f, 0.5f, 0.75f, 1.0f);
+  const auto toValue = 100;
+  nodesManager_->startAnimatingNode(
+      animationId,
+      valueNodeTag,
+      folly::dynamic::object("type", "frames")("frames", frames)(
+          "toValue", toValue)("deferredStart", true),
+      std::nullopt);
+
+  const double t = 12345;
+
+  // Frame 1: both with and without deferredStart, timeDelta=0 → value=0
+  runAnimationFrame(t);
+  EXPECT_EQ(nodesManager_->getValue(valueNodeTag).value(), 0);
+
+  // Frame 2: WITHOUT deferredStart timeDelta=SI → value≈25.
+  // WITH deferredStart the deferred start re-anchored startFrameTimeMs_, so
+  // timeDelta=0 → value=0. This assertion fails without deferredStart.
+  runAnimationFrame(t + SingleFrameIntervalMs);
+  EXPECT_EQ(nodesManager_->getValue(valueNodeTag).value(), 0);
+
+  // Frame 3: now timeDelta=SI from the re-anchored start
+  runAnimationFrame(t + SingleFrameIntervalMs * 2);
+  EXPECT_NEAR(nodesManager_->getValue(valueNodeTag).value(), 25, 0.01);
+
+  // Frame 4
+  runAnimationFrame(t + SingleFrameIntervalMs * 3);
+  EXPECT_NEAR(nodesManager_->getValue(valueNodeTag).value(), 50, 0.01);
+
+  // Complete
+  runAnimationFrame(t + SingleFrameIntervalMs * 5);
+  EXPECT_EQ(nodesManager_->getValue(valueNodeTag).value(), toValue);
+}
+
 } // namespace facebook::react