From d0bd19540ded28cb747a49bf6689dfdfde18d6e0 Mon Sep 17 00:00:00 2001
From: Matt Perry <mattgperry@gmail.com>
Date: Mon, 4 Sep 2023 12:32:05 +0200
Subject: [PATCH] Manually resolving startTime

---
 dev/optimized-appear/resync.html                  | 15 ++++++++-------
 .../waapi/create-accelerated-animation.ts         | 14 +++++++++++++-
 .../animation/interfaces/visual-element-target.ts | 10 ++++++++--
 packages/framer-motion/src/animation/types.ts     |  1 +
 4 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/dev/optimized-appear/resync.html b/dev/optimized-appear/resync.html
index b2902b08e8..ad0dfb366a 100644
--- a/dev/optimized-appear/resync.html
+++ b/dev/optimized-appear/resync.html
@@ -68,15 +68,16 @@
                      */
                     onAnimationStart: () => {
                         frame.postRender(() => {
-                            const box = document.getElementById("box")
+                            frame.postRender(() => {
+                                const box = document.getElementById("box")
 
-                            if (!box) return
+                                if (!box) return
 
-                            const { opacity } = window.getComputedStyle(box)
-
-                            if (parseFloat(opacity) < 0.7) {
-                                showError(box, "Resync failed")
-                            }
+                                const { opacity } = window.getComputedStyle(box)
+                                if (parseFloat(opacity) < 0.65) {
+                                    showError(box, "Resync failed")
+                                }
+                            })
                         })
                     },
                     [optimizedAppearDataAttribute]: "a",
diff --git a/packages/framer-motion/src/animation/animators/waapi/create-accelerated-animation.ts b/packages/framer-motion/src/animation/animators/waapi/create-accelerated-animation.ts
index 7f39304b51..e8c923acb9 100644
--- a/packages/framer-motion/src/animation/animators/waapi/create-accelerated-animation.ts
+++ b/packages/framer-motion/src/animation/animators/waapi/create-accelerated-animation.ts
@@ -1,5 +1,5 @@
 import { EasingDefinition } from "../../../easing/types"
-import { frame, cancelFrame } from "../../../frameloop"
+import { frame, cancelFrame, frameData } from "../../../frameloop"
 import type { VisualElement } from "../../../render/VisualElement"
 import type { MotionValue } from "../../../value"
 import { AnimationPlaybackControls, ValueAnimationOptions } from "../../types"
@@ -136,6 +136,18 @@ export function createAcceleratedAnimation(
         }
     )
 
+    /**
+     * WAAPI animations don't resolve startTime synchronously. But a blocked
+     * thread could delay the startTime resolution by a noticeable amount.
+     * For synching handoff animations with the new Motion animation we want
+     * to ensure startTime is synchronously set.
+     */
+    if (options.syncStart) {
+        animation.startTime = frameData.isProcessing
+            ? frameData.timestamp
+            : performance.now()
+    }
+
     const cancelAnimation = () => animation.cancel()
 
     const safeCancel = () => {
diff --git a/packages/framer-motion/src/animation/interfaces/visual-element-target.ts b/packages/framer-motion/src/animation/interfaces/visual-element-target.ts
index dfcaec86cf..7f27fc44bd 100644
--- a/packages/framer-motion/src/animation/interfaces/visual-element-target.ts
+++ b/packages/framer-motion/src/animation/interfaces/visual-element-target.ts
@@ -2,7 +2,7 @@ import { frame } from "../../frameloop"
 import { transformProps } from "../../render/html/utils/transform"
 import type { AnimationTypeState } from "../../render/utils/animation-state"
 import type { VisualElement } from "../../render/VisualElement"
-import type { TargetAndTransition } from "../../types"
+import type { TargetAndTransition, Transition } from "../../types"
 import { optimizedAppearDataAttribute } from "../optimized-appear/data-id"
 import type { VisualElementAnimationOptions } from "./types"
 import { animateMotionValue } from "./motion-value"
@@ -62,7 +62,12 @@ export function animateTarget(
             continue
         }
 
-        const valueTransition = { delay, elapsed: 0, ...transition }
+        const valueTransition = {
+            delay,
+            syncStart: false,
+            elapsed: 0,
+            ...transition,
+        }
 
         /**
          * If this is the first time a value is being animated, check
@@ -79,6 +84,7 @@ export function animateTarget(
                     value,
                     frame
                 )
+                valueTransition.syncStart = true
             }
         }
 
diff --git a/packages/framer-motion/src/animation/types.ts b/packages/framer-motion/src/animation/types.ts
index 8ad1026ac9..79878f674c 100644
--- a/packages/framer-motion/src/animation/types.ts
+++ b/packages/framer-motion/src/animation/types.ts
@@ -25,6 +25,7 @@ export interface Transition
     type?: "decay" | "spring" | "keyframes" | "tween" | "inertia"
     duration?: number
     autoplay?: boolean
+    syncStart?: boolean
 }
 
 export interface ValueAnimationTransition<V = any>