fix: merge async batches on new dependencies

packages/svelte/src/internal/client/reactivity/batch.js

@@ -171,6 +171,14 @@ export class Batch {
 	 */
 	#new_effects = [];
 
+	/**
+	 * Values that were first read by a reaction while this batch was time travelling
+	 * over an earlier batch that changed them. These reads connect the batches even
+	 * though this batch did not write to the values itself.
+	 * @type {Set<Value>}
+	 */
+	#new_dependencies = new Set();
+
 	/**
 	 * Deferred effects (which run after async work has completed) that are DIRTY
 	 * @type {Set<Effect>}
@@ -482,6 +490,12 @@ export class Batch {
 
 		while (batch !== null) {
 			if (!batch.is_fork) {
+				for (const value of this.#new_dependencies) {
+					if (batch.current.has(value)) {
+						return batch;
+					}
+				}
+
 				// if the batches are connected, break
 				for (const [value, [, is_derived]] of this.current) {
 					if (batch.current.has(value) && !is_derived) {
@@ -500,6 +514,10 @@ export class Batch {
 	 * @param {Batch} batch
 	 */
 	#merge(batch) {
+		for (const value of batch.#new_dependencies) {
+			this.#new_dependencies.add(value);
+		}
+
 		for (const [source, value] of batch.current) {
 			if (!this.previous.has(source) && batch.previous.has(source)) {
 				this.previous.set(source, batch.previous.get(source));
@@ -598,6 +616,28 @@ export class Batch {
 		}
 	}
 
+	/**
+	 * If a reaction discovers a dependency that was changed by an earlier pending
+	 * batch, connect the two batches and expose the current value while traversing.
+	 * The current batch will be merged into the earlier one before it can commit.
+	 * @param {Value} value
+	 */
+	capture_dependency(value) {
+		if (this.is_fork || this.current.has(value)) return;
+
+		var batch = this.#prev;
+
+		while (batch !== null) {
+			if (!batch.is_fork && batch.current.has(value)) {
+				this.#new_dependencies.add(value);
+				batch_values?.set(value, /** @type {[any, boolean]} */ (batch.current.get(value))[0]);
+				return;
+			}
+
+			batch = batch.#prev;
+		}
+	}
+
 	activate() {
 		current_batch = this;
 	}

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

@@ -540,6 +540,14 @@ export function settled() {
 export function get(signal) {
 	var flags = signal.f;
 	var is_derived = (flags & DERIVED) !== 0;
+	var is_new_dependency =
+		batch_values !== null &&
+		!untracking &&
+		((active_reaction !== null &&
+			(active_reaction.deps === null || !includes.call(active_reaction.deps, signal))) ||
+			(active_reaction === null &&
+				active_effect !== null &&
+				(active_effect.f & REACTION_RAN) === 0));
 
 	captured_signals?.add(signal);
 
@@ -699,6 +707,10 @@ export function get(signal) {
 	}
 
 	if (batch_values?.has(signal)) {
+		if (is_new_dependency) {
+			(current_batch ?? previous_batch)?.capture_dependency(signal);
+		}
+
 		return batch_values.get(signal);
 	}
 

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-1/_config.js

@@ -16,20 +16,12 @@ export default test({
 			<button>x</button>
 			<button>y++</button>
 			<button>resolve</button>
-			world
+		`
-		` // if this does not show world - that would also be ok
 		);
 
 		resolve.click();
 		await tick();
-		assert.deepEqual(logs, [
+		assert.deepEqual(logs, ['universe', 'universe', '$effect: universe', '$effect: universe']);
-			'universe',
-			'world',
-			'$effect: world',
-			'$effect: universe',
-			'$effect: universe'
-		]);
-		// assert.deepEqual(logs, ['universe', 'universe', '$effect: universe', '$effect: universe']); // this would also be ok
 		assert.htmlEqual(
 			target.innerHTML,
 			`

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-2/_config.js

@@ -17,13 +17,7 @@ export default test({
 			<button>y++</button>
 			<button>resolve</button>
 			<hr>
-			world
+		`
-			"world"
-			world
-			world
-			world
-			"world"
-		` // if this does not show world "world" world world world "world" - then this would also be ok
 		);
 
 		resolve.click();

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-3/_config.js

@@ -29,13 +29,7 @@ export default test({
 			<button>y++</button>
 			<button>resolve</button>
 			<hr>
-			world
+		`
-			"world"
-			world
-			world
-			world
-			"world"
-		` // if this does not show world "world" world world world "world" - then this would also be ok
 		);
 
 		resolve.click();

packages/svelte/tests/runtime-runes/samples/async-state-new-branch/_config.js

@@ -11,26 +11,19 @@ export default test({
 
 		y.click();
 		await tick();
-		assert.deepEqual(logs, ['universe', 'world', '$effect: world']);
+		assert.deepEqual(logs, ['universe', 'universe']);
 		assert.htmlEqual(
 			target.innerHTML,
 			`
 			<button>x</button>
 			<button>y++</button>
 			<button>resolve</button>
-			world
 		`
 		);
 
 		resolve.click();
 		await tick();
-		assert.deepEqual(logs, [
+		assert.deepEqual(logs, ['universe', 'universe', '$effect: universe', '$effect: universe']);
-			'universe',
-			'world',
-			'$effect: world',
-			'$effect: universe',
-			'$effect: universe'
-		]);
 		assert.htmlEqual(
 			target.innerHTML,
 			`