WIP

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

@@ -31,6 +31,7 @@ export const DESTROYED = 1 << 14;
 export const REACTION_RAN = 1 << 15;
 /** Effect is in the process of getting destroyed. Can be observed in child teardown functions */
 export const DESTROYING = 1 << 25;
+export const EFFECT_LEGACY = 1 << 26;
 
 // Flags exclusive to effects
 /**

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

@@ -23,7 +23,8 @@ import {
 	ERROR_VALUE,
 	WAS_MARKED,
 	MANAGED_EFFECT,
-	REACTION_RAN
+	REACTION_RAN,
+	EFFECT_LEGACY
 } from './constants.js';
 import { old_values } from './reactivity/sources.js';
 import {
@@ -116,9 +117,9 @@ export function push_reaction_value(value) {
  * and until a new dependency is accessed — we track this via `skipped_deps`
  * @type {null | Value[]}
  */
-let new_deps = null;
+export let new_deps = null;
 
-let skipped_deps = 0;
+export let skipped_deps = 0;
 
 /**
  * Tracks writes that the effect it's executed in doesn't listen to yet,
@@ -465,7 +466,7 @@ export function update_effect(effect) {
 		var teardown = update_reaction(effect);
 		effect.teardown = typeof teardown === 'function' ? teardown : null;
 
-		if (is_runes()) {
+		if (is_runes() && (effect.f & EFFECT_LEGACY) === 0) {
 			set_cv(effect, cv);
 		} else {
 			// in legacy mode, prevent the effect re-running immediately

packages/svelte/src/legacy/legacy-client.js

@@ -1,10 +1,18 @@
 /** @import { ComponentConstructorOptions, ComponentType, SvelteComponent, Component } from 'svelte' */
-import { DIRTY, LEGACY_PROPS } from '../internal/client/constants.js';
+/** @import { Derived, Value } from '#client' */
+import { DERIVED, DIRTY, EFFECT_LEGACY, LEGACY_PROPS } from '../internal/client/constants.js';
 import { user_pre_effect } from '../internal/client/reactivity/effects.js';
 import { mutable_source, set } from '../internal/client/reactivity/sources.js';
 import { hydrate, mount, unmount } from '../internal/client/render.js';
-import { active_effect, get } from '../internal/client/runtime.js';
-import { flushSync } from '../internal/client/reactivity/batch.js';
+import {
+	active_effect,
+	get,
+	new_deps,
+	skipped_deps,
+	untracked_writes,
+	write_version
+} from '../internal/client/runtime.js';
+import { flushSync, get_cv } from '../internal/client/reactivity/batch.js';
 import { define_property, is_array } from '../internal/shared/utils.js';
 import * as e from '../internal/client/errors.js';
 import * as w from '../internal/client/warnings.js';
@@ -188,16 +196,37 @@ class Svelte4Component {
  */
 export function run(fn) {
 	user_pre_effect(() => {
-		fn();
 		var effect = /** @type {import('#client').Effect} */ (active_effect);
-		// If the effect is immediately made dirty again, mark it as maybe dirty to emulate legacy behaviour
-		if ((effect.f & DIRTY) !== 0) {
-			let filename = "a file (we can't know which one)";
-			if (DEV) {
-				// @ts-ignore
-				filename = dev_current_component_function?.[FILENAME] ?? filename;
+
+		// we need this to prevent it from re-running
+		effect.f |= EFFECT_LEGACY;
+
+		fn();
+
+		if (untracked_writes !== null) {
+			/** @type {Set<Value>} */
+			const all_deps = new Set([...(effect.deps ?? [].slice(skipped_deps)), ...(new_deps ?? [])]);
+
+			/** @param {Value} dep */
+			const add_deps = (dep) => {
+				all_deps.add(dep);
+
+				if ((dep.f & DERIVED) !== 0) {
+					const deps = /** @type {Derived} */ (dep).deps;
+					if (deps !== null) deps.forEach(add_deps);
+				}
+			};
+
+			for (const dep of all_deps) {
+				if (untracked_writes.includes(dep)) {
+					let filename = "a file (we can't know which one)";
+					if (DEV) {
+						// @ts-ignore
+						filename = dev_current_component_function?.[FILENAME] ?? filename;
+					}
+					w.legacy_recursive_reactive_block(filename);
+				}
 			}
-			w.legacy_recursive_reactive_block(filename);
 		}
 	});
 }