fix: improve transition outro easing

.changeset/brave-shrimps-kiss.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve transition outro easing

packages/svelte/src/internal/client/transitions.js

@@ -256,6 +256,44 @@ function handle_raf(time) {
 	}
 }
 
+/**
+ * @param {{(t: number): number;(t: number): number;(arg0: number): any;}} easing_fn
+ * @param {((t: number, u: number) => string)} css_fn
+ * @param {number} duration
+ * @param {string} direction
+ * @param {boolean} reverse
+ */
+function create_keyframes(easing_fn, css_fn, duration, direction, reverse) {
+	/** @type {Keyframe[]} */
+	const keyframes = [];
+	// We need at least two frames
+	const frame_time = 16.666;
+	const max_duration = Math.max(duration, frame_time);
+	// Have a keyframe every fame for 60 FPS
+	for (let i = 0; i <= max_duration; i += frame_time) {
+		let time;
+		if (i + frame_time > max_duration) {
+			time = 1;
+		} else if (i === 0) {
+			time = 0;
+		} else {
+			time = i / max_duration;
+		}
+		let t = easing_fn(time);
+		if (reverse) {
+			t = 1 - t;
+		}
+		keyframes.push(css_to_keyframe(css_fn(t, 1 - t)));
+	}
+	if (direction === 'out' || reverse) {
+		keyframes.reverse();
+	}
+	return keyframes;
+}
+
+/** @param {number} t */
+const linear = (t) => t;
+
 /**
  * @param {HTMLElement} dom
  * @param {() => import('./types.js').TransitionPayload} init
@@ -286,38 +324,15 @@ function create_transition(dom, init, direction, effect) {
 		const delay = payload.delay ?? 0;
 		const css_fn = payload.css;
 		const tick_fn = payload.tick;
-
-		/** @param {number} t */
-		const linear = (t) => t;
 		const easing_fn = payload.easing || linear;
 
-		/** @type {Keyframe[]} */
-		const keyframes = [];
-
 		if (typeof tick_fn === 'function') {
 			animation = new TickAnimation(tick_fn, duration, delay, direction === 'out');
 		} else {
-			if (typeof css_fn === 'function') {
-				// We need at least two frames
-				const frame_time = 16.666;
-				const max_duration = Math.max(duration, frame_time);
-				// Have a keyframe every fame for 60 FPS
-				for (let i = 0; i <= max_duration; i += frame_time) {
-					let time;
-					if (i + frame_time > max_duration) {
-						time = 1;
-					} else if (i === 0) {
-						time = 0;
-					} else {
-						time = i / max_duration;
-					}
-					const t = easing_fn(time);
-					keyframes.push(css_to_keyframe(css_fn(t, 1 - t)));
-				}
-				if (direction === 'out') {
-					keyframes.reverse();
-				}
-			}
+			const keyframes =
+				typeof css_fn === 'function'
+					? create_keyframes(easing_fn, css_fn, duration, direction, false)
+					: [];
 			animation = dom.animate(keyframes, {
 				duration,
 				endDelay: delay,
@@ -421,6 +436,26 @@ function create_transition(dom, init, direction, effect) {
 			} else {
 				dispatch_event(dom, 'outrostart');
 				if (needs_reverse) {
+					const payload = transition.p;
+					const current_animation = /** @type {Animation} */ (animation);
+					// If we are working with CSS animations, then before we call reverse, we also need to ensure
+					// that we reverse the easing logic. To do this we need to re-create the keyframes so they're
+					// in reverse with easing properly reversed too.
+					if (
+						payload !== null &&
+						payload.css !== undefined &&
+						current_animation.playState === 'idle'
+					) {
+						const duration = payload.duration ?? 300;
+						const css_fn = payload.css;
+						const easing_fn = payload.easing || linear;
+						const keyframes = create_keyframes(easing_fn, css_fn, duration, direction, true);
+						const effect = current_animation.effect;
+						if (effect !== null) {
+							// @ts-ignore
+							effect.setKeyframes(keyframes);
+						}
+					}
 					/** @type {Animation | TickAnimation} */ (animation).reverse();
 				} else {
 					/** @type {Animation | TickAnimation} */ (animation).play();

packages/svelte/tests/animation-helpers.js

@@ -53,11 +53,18 @@ class Animation {
 		this.onfinish = () => {};
 		this.pending = true;
 		this.currentTime = 0;
+		this.playState = 'running';
+		this.effect = {
+			setKeyframes: (/** @type {Keyframe[]} */ keyframes) => {
+				this.#keyframes = keyframes;
+			}
+		};
 	}
 
 	play() {
 		this.#paused = false;
 		raf.animations.add(this);
+		this.playState = 'running';
 		this._update();
 	}
 
@@ -107,6 +114,7 @@ class Animation {
 		if (this.#reversed) {
 			raf.animations.delete(this);
 		}
+		this.playState = 'idle';
 	}
 
 	cancel() {
@@ -118,11 +126,13 @@ class Animation {
 
 	pause() {
 		this.#paused = true;
+		this.playState = 'paused';
 	}
 
 	reverse() {
 		this.#timeline_offset = this.currentTime;
 		this.#reversed = !this.#reversed;
+		this.playState = 'running';
 	}
 }
 

packages/svelte/tests/runtime-legacy/samples/class-shortcut-with-transition/_config.js

@@ -18,7 +18,7 @@ export default test({
 		raf.tick(150);
 		assert.htmlEqual(
 			target.innerHTML,
-			'<p>foo</p><p class="red svelte-1yszte8 border" style="overflow: hidden; opacity: 0; border-top-width: 3.4999399975999683px; border-bottom-width: 3.4999399975999683px;">bar</p>'
+			'<p>foo</p><p class="red svelte-1yszte8 border" style="overflow: hidden; opacity: 0; border-top-width: 0.5000600024000317px; border-bottom-width: 0.5000600024000317px;">bar</p>'
 		);
 		component.open = true;
 		raf.tick(250);