fix: prevent transition action overfiring (#10163)

.changeset/spotty-spiders-compare.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent transition action overfiring

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

@@ -1375,7 +1375,10 @@ function if_block(anchor_node, condition_fn, consequent_fn, alternate_fn) {
 	/** @type {null | import('./types.js').TemplateNode | Array<import('./types.js').TemplateNode>} */
 	let alternate_dom = null;
 	let has_mounted = false;
-	let has_mounted_branch = false;
+	/**
+	 * @type {import("./types.js").EffectSignal | null}
+	 */
+	let current_branch_effect = null;
 
 	const if_effect = render_effect(
 		() => {
@@ -1432,20 +1435,24 @@ function if_block(anchor_node, condition_fn, consequent_fn, alternate_fn) {
 	);
 	// Managed effect
 	const consequent_effect = render_effect(
-		() => {
-			if (consequent_dom !== null) {
+		(
+			/** @type {any} */ _,
+			/** @type {import("./types.js").EffectSignal | null} */ consequent_effect
+		) => {
+			const result = block.v;
+			if (!result && consequent_dom !== null) {
 				remove(consequent_dom);
 				consequent_dom = null;
 			}
-			if (block.v) {
+			if (result && current_branch_effect !== consequent_effect) {
 				consequent_fn(anchor_node);
-				if (!has_mounted_branch) {
+				if (current_branch_effect === null) {
 					// Restore previous fragment so that Svelte continues to operate in hydration mode
 					set_current_hydration_fragment(previous_hydration_fragment);
-					has_mounted_branch = true;
 				}
+				current_branch_effect = consequent_effect;
+				consequent_dom = block.d;
 			}
-			consequent_dom = block.d;
 			block.d = null;
 		},
 		block,
@@ -1454,22 +1461,26 @@ function if_block(anchor_node, condition_fn, consequent_fn, alternate_fn) {
 	block.ce = consequent_effect;
 	// Managed effect
 	const alternate_effect = render_effect(
-		() => {
-			if (alternate_dom !== null) {
+		(
+			/** @type {any} */ _,
+			/** @type {import("./types.js").EffectSignal | null} */ alternate_effect
+		) => {
+			const result = block.v;
+			if (result && alternate_dom !== null) {
 				remove(alternate_dom);
 				alternate_dom = null;
 			}
-			if (!block.v) {
+			if (!result && current_branch_effect !== alternate_effect) {
 				if (alternate_fn !== null) {
 					alternate_fn(anchor_node);
 				}
-				if (!has_mounted_branch) {
+				if (current_branch_effect === null) {
 					// Restore previous fragment so that Svelte continues to operate in hydration mode
 					set_current_hydration_fragment(previous_hydration_fragment);
-					has_mounted_branch = true;
 				}
+				current_branch_effect = alternate_effect;
+				alternate_dom = block.d;
 			}
-			alternate_dom = block.d;
 			block.d = null;
 		},
 		block,

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

@@ -342,9 +342,13 @@ function execute_signal_fn(signal) {
 	try {
 		let res;
 		if (is_render_effect) {
-			res = /** @type {(block: import('./types.js').Block) => V} */ (init)(
-				/** @type {import('./types.js').Block} */ (signal.b)
-			);
+			res =
+				/** @type {(block: import('./types.js').Block, signal: import('./types.js').Signal) => V} */ (
+					init
+				)(
+					/** @type {import('./types.js').Block} */ (signal.b),
+					/** @type {import('./types.js').Signal} */ (signal)
+				);
 		} else {
 			res = /** @type {() => V} */ (init)();
 		}

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

@@ -31,7 +31,6 @@ const DELAY_NEXT_TICK = Number.MIN_SAFE_INTEGER;
 
 /** @type {undefined | number} */
 let active_tick_ref = undefined;
-let skip_mount_intro = false;
 
 /**
  * @template T
@@ -496,7 +495,7 @@ export function bind_transition(dom, get_transition_fn, props_fn, direction, glo
 				can_show_intro_on_mount = true;
 			} else if (transition_block.t === IF_BLOCK) {
 				transition_block.r = if_block_transition;
-				if (can_show_intro_on_mount && !skip_mount_intro) {
+				if (can_show_intro_on_mount) {
 					/** @type {import('./types.js').Block | null} */
 					let if_block = transition_block;
 					while (if_block.t === IF_BLOCK) {
@@ -511,14 +510,13 @@ export function bind_transition(dom, get_transition_fn, props_fn, direction, glo
 				}
 			}
 			if (!can_apply_lazy_transitions && can_show_intro_on_mount) {
-				can_show_intro_on_mount = !skip_mount_intro && transition_block.e !== null;
+				can_show_intro_on_mount = transition_block.e !== null;
 			}
 			if (can_show_intro_on_mount || !global) {
 				can_apply_lazy_transitions = true;
 			}
 		} else if (transition_block.t === ROOT_BLOCK && !can_apply_lazy_transitions) {
-			can_show_intro_on_mount =
-				!skip_mount_intro && (transition_block.e !== null || transition_block.i);
+			can_show_intro_on_mount = transition_block.e !== null || transition_block.i;
 		}
 		transition_block = transition_block.p;
 	}
@@ -527,14 +525,11 @@ export function bind_transition(dom, get_transition_fn, props_fn, direction, glo
 	let transition;
 
 	effect(() => {
+		let already_mounted = false;
 		if (transition !== undefined) {
+			already_mounted = true;
 			// Destroy any existing transitions first
-			try {
-				skip_mount_intro = true;
-				transition.x();
-			} finally {
-				skip_mount_intro = false;
-			}
+			transition.x();
 		}
 		const transition_fn = get_transition_fn();
 		/** @param {DOMRect} [from] */
@@ -557,7 +552,7 @@ export function bind_transition(dom, get_transition_fn, props_fn, direction, glo
 		const is_intro = direction === 'in';
 		const show_intro = can_show_intro_on_mount && (is_intro || direction === 'both');
 
-		if (show_intro) {
+		if (show_intro && !already_mounted) {
 			transition.p = transition.i();
 		}
 
@@ -565,7 +560,7 @@ export function bind_transition(dom, get_transition_fn, props_fn, direction, glo
 			destroy_signal(effect);
 			dom.inert = false;
 
-			if (show_intro) {
+			if (show_intro && !already_mounted) {
 				transition.in();
 			}
 
@@ -662,7 +657,8 @@ function if_block_transition(transition) {
 			const c = /** @type {Set<import('./types.js').Transition>} */ (consequent_transitions);
 			c.delete(transition);
 			if (c.size === 0) {
-				execute_effect(/** @type {import('./types.js').EffectSignal} */ (block.ce));
+				const consequent_effect = block.ce;
+				execute_effect(/** @type {import('./types.js').EffectSignal} */ (consequent_effect));
 			}
 		});
 	} else {
@@ -675,7 +671,8 @@ function if_block_transition(transition) {
 			const a = /** @type {Set<import('./types.js').Transition>} */ (alternate_transitions);
 			a.delete(transition);
 			if (a.size === 0) {
-				execute_effect(/** @type {import('./types.js').EffectSignal} */ (block.ae));
+				const alternate_effect = block.ae;
+				execute_effect(/** @type {import('./types.js').EffectSignal} */ (alternate_effect));
 			}
 		});
 	}

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

@@ -99,7 +99,11 @@ export type ComputationSignal<V = unknown> = {
 	/** The types that the signal represent, as a bitwise value */
 	f: SignalFlags;
 	/** init: The function that we invoke for effects and computeds */
-	i: null | (() => V) | (() => void | (() => void)) | ((b: Block) => void | (() => void));
+	i:
+		| null
+		| (() => V)
+		| (() => void | (() => void))
+		| ((b: Block, s: Signal) => void | (() => void));
 	/** references: Anything that a signal owns */
 	r: null | ComputationSignal[];
 	/** value: The latest value for this signal, doubles as the teardown for effects */

packages/svelte/tests/runtime-runes/samples/dynamic-transition/_config.js

@@ -20,6 +20,12 @@ export default test({
 			b2.click();
 		});
 
+		assert.deepEqual(log, ['transition 2']);
+
+		flushSync(() => {
+			b1.click();
+		});
+
 		assert.deepEqual(log, ['transition 2', 'transition 1']);
 	}
 });