fix: improve animation heuristics

.changeset/soft-tigers-wink.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve animation transition heuristics

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

@@ -25,7 +25,6 @@ import {
 	mutable_source,
 	push_destroy_fn,
 	render_effect,
-	schedule_task,
 	set_signal_value,
 	source
 } from './runtime.js';
@@ -486,6 +485,17 @@ function reconcile_tracked_array(
 					key = is_computed_key ? keys[a] : item;
 					map_set(item_index, key, a);
 				}
+				// If keys are animated, we need to do updates before actual moves
+				if (is_animated) {
+					for (b = start; b <= a_end; ++b) {
+						a = map_get(item_index, /** @type {V} */ (a_blocks[b].k));
+						if (a !== undefined) {
+							item = array[a];
+							block = a_blocks[b];
+							update_each_item_block(block, item, a, flags);
+						}
+					}
+				}
 				for (b = start; b <= a_end; ++b) {
 					a = map_get(item_index, /** @type {V} */ (a_blocks[b].k));
 					block = a_blocks[b];
@@ -501,22 +511,9 @@ function reconcile_tracked_array(
 				if (pos === MOVED_BLOCK) {
 					mark_lis(sources);
 				}
-				// If keys are animated, we need to do updates before actual moves
-				var should_create;
-				if (is_animated) {
-					var i = b_length;
-					while (i-- > 0) {
-						b_end = i + start;
-						a = sources[i];
-						if (pos === MOVED_BLOCK) {
-							block = b_blocks[b_end];
-							item = array[b_end];
-							update_each_item_block(block, item, b_end, flags);
-						}
-					}
-				}
 				var last_block;
 				var last_sibling;
+				var should_create;
 				while (b_length-- > 0) {
 					b_end = b_length + start;
 					a = sources[b_length];
@@ -715,7 +712,7 @@ function update_each_item_block(block, item, index, type) {
 	// Handle each item animations
 	const each_animation = block.a;
 	if (transitions !== null && (type & EACH_KEYED) !== 0 && each_animation !== null) {
-		each_animation(block, transitions, index, index_is_reactive);
+		each_animation(block, transitions);
 	}
 	if (index_is_reactive) {
 		set_signal_value(/** @type {import('./types.js').Signal<number>} */ (block.i), index);

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

@@ -37,6 +37,7 @@ let current_scheduler_mode = FLUSH_MICROTASK;
 // Used for handling scheduling
 let is_micro_task_queued = false;
 let is_task_queued = false;
+let is_raf_queued = false;
 // Used for $inspect
 export let is_batching_effect = false;
 
@@ -51,7 +52,7 @@ let current_queued_effects = [];
 /** @type {Array<() => void>} */
 let current_queued_tasks = [];
 /** @type {Array<() => void>} */
-let current_queued_microtasks = [];
+let current_raf_tasks = [];
 let flush_count = 0;
 // Handle signal reactivity tree dependencies and consumer
 
@@ -585,11 +586,6 @@ function flush_queued_effects(effects) {
 
 function process_microtask() {
 	is_micro_task_queued = false;
-	if (current_queued_microtasks.length > 0) {
-		const tasks = current_queued_microtasks.slice();
-		current_queued_microtasks = [];
-		run_all(tasks);
-	}
 	if (flush_count > 101) {
 		return;
 	}
@@ -636,6 +632,13 @@ function process_task() {
 	run_all(tasks);
 }
 
+function process_raf_task() {
+	is_raf_queued = false;
+	const tasks = current_raf_tasks.slice();
+	current_raf_tasks = [];
+	run_all(tasks);
+}
+
 /**
  * @param {() => void} fn
  * @returns {void}
@@ -652,12 +655,12 @@ export function schedule_task(fn) {
  * @param {() => void} fn
  * @returns {void}
  */
-export function schedule_microtask(fn) {
-	if (!is_micro_task_queued) {
-		is_micro_task_queued = true;
-		queueMicrotask(process_microtask);
+export function schedule_raf_task(fn) {
+	if (!is_raf_queued) {
+		is_raf_queued = true;
+		requestAnimationFrame(process_raf_task);
 	}
-	current_queued_microtasks.push(fn);
+	current_raf_tasks.push(fn);
 }
 
 /**
@@ -720,8 +723,8 @@ export function flushSync(fn) {
 		if (current_queued_pre_and_render_effects.length > 0 || effects.length > 0) {
 			flushSync();
 		}
-		if (is_micro_task_queued) {
-			process_microtask();
+		if (is_raf_queued) {
+			process_raf_task();
 		}
 		if (is_task_queued) {
 			process_task();

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

@@ -21,7 +21,7 @@ import {
 	managed_effect,
 	managed_pre_effect,
 	mark_subtree_inert,
-	schedule_microtask,
+	schedule_raf_task,
 	untrack
 } from './runtime.js';
 import { raf } from './timing.js';
@@ -646,7 +646,8 @@ function each_item_transition(transition) {
 			transitions.delete(transition);
 			if (transition.r !== 'key') {
 				for (let other of transitions) {
-					if (other.r === 'key' || other.r === 'in') {
+					const type = other.r;
+					if (type === 'key' || type === 'in') {
 						transitions.delete(other);
 					}
 				}
@@ -664,26 +665,18 @@ function each_item_transition(transition) {
  *
  * @param {import('./types.js').EachItemBlock} block
  * @param {Set<import('./types.js').Transition>} transitions
- * @param {number} index
- * @param {boolean} index_is_reactive
  */
-function each_item_animate(block, transitions, index, index_is_reactive) {
-	let prev_index = block.i;
-	if (index_is_reactive) {
-		prev_index = /** @type {import('./types.js').Signal<number>} */ (prev_index).v;
-	}
-	const items = block.p.v;
-	if (prev_index !== index && /** @type {number} */ (index) < items.length) {
-		const from_dom = /** @type {Element} */ (get_first_element(block));
-		const from = from_dom.getBoundingClientRect();
-		// Cancel any existing key transitions
-		for (const transition of transitions) {
-			if (transition.r === 'key') {
-				transition.c();
-			}
+function each_item_animate(block, transitions) {
+	const from_dom = /** @type {Element} */ (get_first_element(block));
+	const from = from_dom.getBoundingClientRect();
+	// Cancel any existing key transitions
+	for (const transition of transitions) {
+		const type = transition.r;
+		if (type === 'key') {
+			transition.c();
 		}
-		schedule_microtask(() => {
-			trigger_transitions(transitions, 'key', from);
-		});
 	}
+	schedule_raf_task(() => {
+		trigger_transitions(transitions, 'key', from);
+	});
 }

packages/svelte/src/internal/client/types.d.ts

@@ -288,14 +288,7 @@ export type EachBlock = {
 
 export type EachItemBlock = {
 	/** transition */
-	a:
-		| null
-		| ((
-				block: EachItemBlock,
-				transitions: Set<Transition>,
-				index: number,
-				index_is_reactive: boolean
-		  ) => void);
+	a: null | ((block: EachItemBlock, transitions: Set<Transition>) => void);
 	/** dom */
 	d: null | TemplateNode | Array<TemplateNode>;
 	/** effect */

packages/svelte/tests/runtime-legacy/samples/dynamic-element-animation/_config.js

@@ -53,9 +53,9 @@ export default test({
 
 		divs = target.querySelectorAll('div');
 		assert.ok(~divs[0].style.transform);
-		assert.equal(divs[1].style.transform, '');
-		assert.equal(divs[2].style.transform, '');
-		assert.equal(divs[3].style.transform, '');
+		assert.equal(divs[1].style.transform, 'translate(1px, 0px)');
+		assert.equal(divs[2].style.transform, 'translate(1px, 0px)');
+		assert.equal(divs[3].style.transform, 'translate(1px, 0px)');
 		assert.ok(~divs[4].style.transform);
 
 		raf.tick(100);