chore: lots of async tests (#17997)

extracting this from #17971 because even if that PR is not merged these
tests show that there's still a lot of cases where we're buggy,
particularly around forking and new branches

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-1/_config.js

@@ -0,0 +1,107 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a_b, a_c, b_d, shift, pop] = target.querySelectorAll('button');
+
+		a_b.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		b_d.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 1 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 2 | b 1 | c 1 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 2 | b 2 | c 1 | d 1
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-1/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let d = $state(0);
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {await delay(a)} | b {await delay(b)} | c {c} | d {d}
+<button onclick={() => {a++;b++;}}>
+	a and b
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => {b++;d++;}}>
+	b and d
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-2/_config.js

@@ -0,0 +1,82 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		await tick();
+		const [a_b, a_c, b_d, shift, pop] = target.querySelectorAll('button');
+
+		a_b.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		b_d.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click(); // second b resolved, blocked on first batch because a still pending
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		for (let i = 0; i < 3; i++) {
+			pop.click(); // second a resolved, first a/b now obsolete; empty queue
+			await tick();
+			assert.htmlEqual(
+				target.innerHTML,
+				`
+				a 2 | b 2 | c 1 | d 1
+				<button>a and b</button>
+				<button>a and c</button>
+				<button>b and d</button>
+				<button>shift</button>
+				<button>pop</button>
+			`
+			);
+		}
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-2/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let d = $state(0);
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {await delay(a)} | b {await delay(b)} | c {c} | d {d}
+<button onclick={() => {a++;b++;}}>
+	a and b
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => {b++;d++;}}>
+	b and d
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-3/_config.js

@@ -0,0 +1,108 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		await tick();
+		const [a_b, a_c, b_d, shift, pop] = target.querySelectorAll('button');
+
+		a_b.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		b_d.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // first a resolved, still pending: [b, a, b]
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click(); // second b resolved, still pending: [b, a]
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // first b resolved, first + last batch settled, still pending: [a]
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 0 | d 1
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // all resolved
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 2 | b 2 | c 1 | d 1
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-3/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let d = $state(0);
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {await delay(a)} | b {await delay(b)} | c {c} | d {d}
+<button onclick={() => {a++;b++;}}>
+	a and b
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => {b++;d++;}}>
+	b and d
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-4/_config.js

@@ -0,0 +1,110 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		await tick();
+		const [a_b, a_c, b_d, shift, pop] = target.querySelectorAll('button');
+
+		a_b.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		b_d.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // first a resolved, still pending: [b, a, b]
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click(); // second b resolved, still pending: [b, a]
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click(); // second a resolved, first a/b now obsolete
+		// TODO would be nice to show final result here already, right now it doesn't because
+		// we have no handle on the already resolved first a anymore
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // queue empty
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 2 | b 2 | c 1 | d 1
+			<button>a and b</button>
+			<button>a and c</button>
+			<button>b and d</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-4/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let d = $state(0);
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {await delay(a)} | b {await delay(b)} | c {c} | d {d}
+<button onclick={() => {a++;b++;}}>
+	a and b
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => {b++;d++;}}>
+	b and d
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/_config.js

@@ -0,0 +1,118 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, c, shift, pop] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		// how it's on main
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 1 | d 1
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 1 | d 3
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		// how it's on https://github.com/sveltejs/svelte/pull/17971
+		// shift.click();
+		// await tick();
+		// assert.htmlEqual(
+		// 	target.innerHTML,
+		// 	`
+		// 	a 0 | b 0 | c 0 | d 0
+		// 	<button>a++</button>
+		// 	<button>c++</button>
+		// 	<button>shift</button>
+		// 	<button>pop</button>
+		// `
+		// );
+
+		// shift.click();
+		// await tick();
+		// assert.htmlEqual(
+		// 	target.innerHTML,
+		// 	`
+		// 	a 1 | b 2 | c 0 | d 2
+		// 	<button>a++</button>
+		// 	<button>c++</button>
+		// 	<button>shift</button>
+		// 	<button>pop</button>
+		// `
+		// );
+
+		// shift.click();
+		// await tick();
+		// assert.htmlEqual(
+		// 	target.innerHTML,
+		// 	`
+		// 	a 1 | b 2 | c 1 | d 3
+		// 	<button>a++</button>
+		// 	<button>c++</button>
+		// 	<button>shift</button>
+		// 	<button>pop</button>
+		// `
+		// );
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	let a = $state(0);
+	let b = $derived(await delay(a * 2));
+	let c = $state(0);
+	let d = $derived(await delay(b + c));
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value; 
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {a} | b {b} | c {c} | d {d}
+<button onclick={() => {a++;}}>
+	a++
+</button>
+<button onclick={() => {c++;}}>
+	c++
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/_config.js

@@ -0,0 +1,76 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, c, shift, pop] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		// Although the second batch is eventually connected to the first one, we can't see that
+		// at this point yet and so the second one flushes right away.
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 1 | d 1
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 1 | d 1
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 1 | d 3
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	let a = $state(0);
+	let b = $derived(await delay(a * 2));
+	let c = $state(0);
+	let d = $derived(await delay(b + c));
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value; 
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {a} | b {b} | c {c} | d {d}
+<button onclick={() => {a++;}}>
+	a++
+</button>
+<button onclick={() => {c++;}}>
+	c++
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/_config.js

@@ -0,0 +1,129 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, c, shift, pop] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // schedules second step of first batch and schedules rerun of second batch
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		// how it's on main
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 0 | d 2
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // obsolete second batch promise (already rejected)
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 0 | d 2
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // first batch resolves
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 1 | d 3
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		// how it's on https://github.com/sveltejs/svelte/pull/17971
+		// pop.click(); // second batch resolves but knows it needs to wait on first batch
+		// await tick();
+		// assert.htmlEqual(
+		// 	target.innerHTML,
+		// 	`
+		// 	a 0 | b 0 | c 0 | d 0
+		// 	<button>a++</button>
+		// 	<button>c++</button>
+		// 	<button>shift</button>
+		// 	<button>pop</button>
+		// `
+		// );
+
+		// shift.click(); // obsolete second batch promise (already rejected)
+		// await tick();
+		// assert.htmlEqual(
+		// 	target.innerHTML,
+		// 	`
+		// 	a 0 | b 0 | c 0 | d 0
+		// 	<button>a++</button>
+		// 	<button>c++</button>
+		// 	<button>shift</button>
+		// 	<button>pop</button>
+		// `
+		// );
+
+		// shift.click(); // first batch resolves, with it second can now resolve as well
+		// await tick();
+		// assert.htmlEqual(
+		// 	target.innerHTML,
+		// 	`
+		// 	a 1 | b 2 | c 1 | d 3
+		// 	<button>a++</button>
+		// 	<button>c++</button>
+		// 	<button>shift</button>
+		// 	<button>pop</button>
+		// `
+		// );
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	let a = $state(0);
+	let b = $derived(await delay(a * 2));
+	let c = $state(0);
+	let d = $derived(await delay(b + c));
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value; 
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {a} | b {b} | c {c} | d {d}
+<button onclick={() => {a++;}}>
+	a++
+</button>
+<button onclick={() => {c++;}}>
+	c++
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-1/_config.js

@@ -0,0 +1,34 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a_b_fork, a_c, shift, pop, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+
+		a_b_fork.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0');
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0');
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 0 | c 1');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 0 | c 1');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 0 | c 1');
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-1/main.svelte

@@ -0,0 +1,27 @@
+<script>
+	import { fork } from 'svelte';
+
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<p>a {await delay(a)} | b {await delay(b)} | c {c}</p>
+
+<button onclick={() => {f = fork(() => {a++;b++;});}}>
+	a and b (fork)
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit fork</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-2/_config.js

@@ -0,0 +1,42 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a_b_fork, a_c, b_d, shift, pop, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+
+		a_b_fork.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		b_d.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 1 | c 0 | d 1');
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-2/main.svelte

@@ -0,0 +1,31 @@
+<script>
+	import { fork } from 'svelte';
+
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let d = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<p>a {await delay(a)} | b {await delay(b)} | c {c} | d {d}</p>
+
+<button onclick={() => {f = fork(() => {a++;b++;});}}>
+	a and b (fork)
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => {b++;d++;}}>
+	b and d
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit fork</button>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-3/_config.js

@@ -0,0 +1,43 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true,
+	async test({ assert, target }) {
+		await tick();
+		const [a_b_fork, a_c, b_d, shift, pop, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+
+		a_b_fork.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		a_c.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		b_d.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 0 | c 0 | d 0');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 0 | c 1 | d 0');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-3/main.svelte

@@ -0,0 +1,31 @@
+<script>
+	import { fork } from 'svelte';
+
+	let a = $state(0);
+	let b = $state(0);
+	let c = $state(0);
+	let d = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<p>a {await delay(a)} | b {await delay(b)} | c {c} | d {d}</p>
+
+<button onclick={() => {f = fork(() => {a++;b++;});}}>
+	a and b (fork)
+</button>
+<button onclick={() => {a++;c++;}}>
+	a and c
+</button>
+<button onclick={() => {b++;d++;}}>
+	b and d
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit fork</button>

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

@@ -0,0 +1,7 @@
+<script>
+	let { x } = $props();
+	console.log(x);
+	$effect(() => console.log('$effect: '+ x))
+</script>
+
+{x}

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

@@ -0,0 +1,38 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target, logs }) {
+		const [x, y, resolve] = target.querySelectorAll('button');
+
+		x.click();
+		await tick();
+
+		y.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+		` // if this shows world world - that would also be ok
+		);
+
+		resolve.click();
+		await tick();
+		assert.deepEqual(logs, ['universe', 'universe', '$effect: universe', '$effect: universe']);
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			universe
+			universe
+			universe
+		`
+		);
+	}
+});

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

@@ -0,0 +1,28 @@
+<script>
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let deferred = [];
+
+	function delay(s) {
+		const d = Promise.withResolvers();
+		deferred.push(() => d.resolve(s))
+		return d.promise;
+	}
+</script>
+
+<button onclick={() => x = 'universe'}>x</button>
+
+<button onclick={() => y++}>y++</button>
+
+<button onclick={() => deferred.shift()()}>resolve</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+{#if y > 0}
+	<Child {x} />
+{/if}

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

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

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

@@ -0,0 +1,49 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		const [x, y, resolve] = target.querySelectorAll('button');
+
+		x.click();
+		await tick();
+
+		y.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<hr>
+		` // if this shows world world "world" world world world "world" - then this would also be ok
+		);
+
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

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

@@ -0,0 +1,30 @@
+<script>
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let deferred = [];
+
+	function delay(s) {
+		const d = Promise.withResolvers();
+		deferred.push(() => d.resolve(s))
+		return d.promise;
+	}
+</script>
+
+<button onclick={() => x = 'universe'}>x</button>
+
+<button onclick={() => y++}>y++</button>
+
+<button onclick={() => deferred.shift()()}>resolve</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child {x} />
+{/if}

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

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

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

@@ -0,0 +1,61 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		const [x, y, resolve] = target.querySelectorAll('button');
+
+		x.click();
+		await tick();
+
+		y.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<hr>
+		`
+		);
+
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<hr>
+		` // if this shows world world "world" world world world "world" - then this would also be ok
+		);
+
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

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

@@ -0,0 +1,34 @@
+<script>
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	function delay2(value) {
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (x = 'universe')}>x</button>
+
+<button onclick={() => y++}>y++</button>
+<button onclick={() => deferred.pop()?.()}>resolve</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child x={await delay2(x)} />
+{/if}
+ 

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-all-combinations/Child.svelte

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

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

@@ -0,0 +1,171 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO more combinations pass on https://github.com/sveltejs/svelte/pull/17971
+	timeout: 20_000,
+	async test({ assert, target }) {
+		const [x, fork_x, y, fork_y, shift, pop, commit_x, commit_y, reset] =
+			target.querySelectorAll('button');
+
+		const initial = `
+			<button>x</button>
+			<button>x (fork)</button>
+			<button>y++</button>
+			<button>y++ (fork)</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit x</button>
+			<button>commit y</button>
+			<button>reset</button>
+			<hr>
+		`;
+
+		const final = `
+			<button>x</button>
+			<button>x (fork)</button>
+			<button>y++</button>
+			<button>y++ (fork)</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit x</button>
+			<button>commit y</button>
+			<button>reset</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`;
+
+		/** @param {HTMLElement} button */
+		async function click(button) {
+			button.click();
+			await tick();
+		}
+
+		/**
+		 * Generate all permutations of an array.
+		 * @param {HTMLElement[]} actions
+		 * @returns {HTMLElement[][]}
+		 */
+		function permutations(actions) {
+			if (actions.length <= 1) return [actions];
+
+			/** @type {HTMLElement[][]} */
+			const result = [];
+
+			for (let i = 0; i < actions.length; i++) {
+				const head = actions[i];
+				const rest = actions.slice(0, i).concat(actions.slice(i + 1));
+				for (const tail of permutations(rest)) {
+					result.push([head, ...tail]);
+				}
+			}
+
+			return result;
+		}
+
+		/**
+		 * Keep only valid orders where fork commits happen after their fork action.
+		 * @param {HTMLElement[]} order
+		 */
+		function is_valid_order(order) {
+			const x_fork_index = order.indexOf(fork_x);
+			const commit_x_index = order.indexOf(commit_x);
+			if (commit_x_index !== -1 && (x_fork_index === -1 || commit_x_index < x_fork_index)) {
+				return false;
+			}
+
+			const y_fork_index = order.indexOf(fork_y);
+			const commit_y_index = order.indexOf(commit_y);
+			if (commit_y_index !== -1 && (y_fork_index === -1 || commit_y_index < y_fork_index)) {
+				return false;
+			}
+
+			return true;
+		}
+
+		/**
+		 * Four control scenarios:
+		 * - x direct, y direct
+		 * - x direct, y via fork+commit
+		 * - x via fork+commit, y direct
+		 * - x via fork+commit, y via fork+commit
+		 */
+		const control_scenarios = [
+			[x, y],
+			[x, fork_y, commit_y],
+			[fork_x, commit_x, y],
+			[fork_x, commit_x, fork_y, commit_y]
+		];
+
+		const control_orders = control_scenarios.flatMap((scenario) =>
+			permutations(scenario).filter(is_valid_order)
+		);
+
+		/**
+		 * All shift/pop combinations for draining async work.
+		 * We click three times because this scenario can queue up to 3 deferred resolutions.
+		 */
+		const resolve_orders = [
+			[shift, shift, shift],
+			[shift, pop, pop],
+			[pop, shift, shift],
+			[pop, pop, pop]
+		];
+
+		for (const controls of control_orders) {
+			for (const resolves of resolve_orders) {
+				for (const action of controls) {
+					await click(action);
+				}
+
+				for (const action of resolves) {
+					await click(action);
+				}
+
+				const failure_msg = `Failed for: ${controls
+					.map((btn) => btn.textContent)
+					.concat(...resolves.map((btn) => btn.textContent))
+					.join(', ')}`;
+				assert.htmlEqual(target.innerHTML, final, failure_msg);
+
+				await click(reset);
+				assert.htmlEqual(target.innerHTML, initial, failure_msg);
+			}
+		}
+
+		const other_scenarios = [
+			[x, shift, y, shift, shift],
+			[x, shift, y, pop, pop],
+			[fork_x, shift, y, shift, commit_x, shift],
+			[fork_x, shift, y, pop, commit_x, pop],
+			[y, shift, x, shift, shift],
+			[y, shift, x, pop, pop],
+			[fork_y, shift, x, shift, commit_y, shift],
+			[fork_y, shift, x, pop, commit_y, pop]
+		];
+
+		for (const scenario of other_scenarios) {
+			for (const action of scenario) {
+				await click(action);
+			}
+
+			const failure_msg = `Failed for: ${scenario.map((btn) => btn.textContent).join(', ')}`;
+			assert.htmlEqual(target.innerHTML, final, failure_msg);
+
+			await click(reset);
+			assert.htmlEqual(target.innerHTML, initial, failure_msg);
+		}
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-all-combinations/main.svelte

@@ -0,0 +1,41 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let fx;
+	let fy;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	function delay2(value) {
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (x = 'universe')}>x</button>
+<button onclick={() => (fx = fork(() => {x = 'universe';}))}>x (fork)</button>
+<button onclick={() => y++}>y++</button>
+<button onclick={() => (fy = fork(() => {y++;}))}>y++ (fork)</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => fx.commit()}>commit x</button>
+<button onclick={() => fy.commit()}>commit y</button>
+<button onclick={() => {x = 'world'; y = 0;}}>reset</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child x={await delay2(x)} />
+{/if}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-1/Child.svelte

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

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

@@ -0,0 +1,90 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		const [x, y, shift, pop, commit] = target.querySelectorAll('button');
+
+		x.click();
+		await tick();
+
+		y.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+		`
+		);
+
+		shift.click();
+		await tick();
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-1/main.svelte

@@ -0,0 +1,36 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	function delay2(value) {
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (f = fork(() => {x = 'universe';}))}>x</button>
+<button onclick={() => y++}>y++</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child x={await delay2(x)} />
+{/if}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-2/Child.svelte

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

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

@@ -0,0 +1,71 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		const [x, y, shift, pop, commit] = target.querySelectorAll('button');
+
+		y.click();
+		await tick();
+
+		x.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		shift.click();
+		await tick();
+		shift.click();
+		await tick();
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-2/main.svelte

@@ -0,0 +1,36 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	function delay2(value) {
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (f = fork(() => {x = 'universe';}))}>x</button>
+<button onclick={() => y++}>y++</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child x={await delay2(x)} />
+{/if}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-3/Child.svelte

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

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

@@ -0,0 +1,70 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [x, y, shift, pop, commit] = target.querySelectorAll('button');
+
+		y.click();
+		await tick();
+
+		x.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		pop.click();
+		await tick();
+		pop.click();
+		await tick();
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>shift</button>
+			<button>pop</button>
+			<button>commit</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-3/main.svelte

@@ -0,0 +1,36 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	function delay2(value) {
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (f = fork(() => {x = 'universe';}))}>x</button>
+<button onclick={() => y++}>y++</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child x={await delay2(x)} />
+{/if}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-4/Child.svelte

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-4/_config.js

@@ -0,0 +1,76 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		const [x, y, resolve, commit] = target.querySelectorAll('button');
+
+		x.click();
+		await tick();
+
+		y.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			<hr>
+			world
+			"world"
+			world
+			world
+			world
+			"world"
+		`
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			<hr>
+			world
+			"world"
+			world
+			world
+			world
+			"world"
+		`
+		);
+
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-4/main.svelte

@@ -0,0 +1,32 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (f = fork(() => {x = 'universe';}))}>x</button>
+
+<button onclick={() => y++}>y++</button>
+<button onclick={() => deferred.pop()?.()}>resolve</button>
+<button onclick={() => f.commit()}>commit</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child {x} />
+{/if}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-5/Child.svelte

@@ -0,0 +1,11 @@
+<script>
+	let { x } = $props();
+</script>
+
+<!-- checks direct source, indirect derived, block effect, async effect -->
+{x}
+{JSON.stringify(x)}
+{#if x === 'universe'}universe{:else}world{/if}
+{#if JSON.stringify(x) === '"universe"'}universe{:else}world{/if}
+{await Promise.resolve(x)}
+{await Promise.resolve(JSON.stringify(x))}

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-5/_config.js

@@ -0,0 +1,85 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip: true, // TODO works on https://github.com/sveltejs/svelte/pull/17971
+	async test({ assert, target }) {
+		const [x, y, resolve, commit] = target.querySelectorAll('button');
+
+		x.click();
+		await tick();
+
+		y.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			<hr>
+		`
+		);
+
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			<hr>
+			world
+			"world"
+			world
+			world
+			world
+			"world"
+		`
+		);
+
+		resolve.click();
+		await tick();
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>x</button>
+			<button>y++</button>
+			<button>resolve</button>
+			<button>commit</button>
+			universe
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+			<hr>
+			universe
+			"universe"
+			universe
+			universe
+			universe
+			"universe"
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-new-branch-fork-5/main.svelte

@@ -0,0 +1,36 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state('world');
+	let y = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (value !== 'universe') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	function delay2(value) {
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => (f = fork(() => {x = 'universe';}))}>x</button>
+
+<button onclick={() => y++}>y++</button>
+<button onclick={() => deferred.pop()?.()}>resolve</button>
+<button onclick={() => f.commit()}>commit</button>
+
+{#if x === 'universe'}
+	{await delay(x)}
+	<Child {x} />
+{/if}
+
+<hr>
+
+{#if y > 0}
+	<Child x={await delay2(x)} />
+{/if}

packages/svelte/tests/suite.ts

@@ -4,6 +4,7 @@ import { it } from 'vitest';
 export interface BaseTest {
 	skip?: boolean;
 	solo?: boolean;
+	timeout?: number;
 }
 
 /**
@@ -30,7 +31,7 @@ export function suite<Test extends BaseTest>(fn: (config: Test, test_dir: string
 			await for_each_dir<Test>(cwd, samples_dir, (config, dir) => {
 				let it_fn = config.skip ? it.skip : config.solo ? it.only : it;
 
-				it_fn(dir, () => fn(config, `${cwd}/${samples_dir}/${dir}`));
+				it_fn(dir, { timeout: config.timeout }, () => fn(config, `${cwd}/${samples_dir}/${dir}`));
 			});
 		}
 	};
@@ -57,7 +58,7 @@ export function suite_with_variants<Test extends BaseTest, Variants extends stri
 					const solo = config.solo;
 					let it_fn = skip ? it.skip : solo ? it.only : it;
 
-					it_fn(`${dir} (${variant})`, async () => {
+					it_fn(`${dir} (${variant})`, { timeout: config.timeout }, async () => {
 						if (!called_common) {
 							called_common = true;
 							common = await common_setup(config, `${cwd}/${samples_dir}/${dir}`);