From b915fa0c3b60a847ae3505fea83681a0709fad9a Mon Sep 17 00:00:00 2001
From: Elliott Johnson <hello+git@ell.iott.dev>
Date: Fri, 9 Oct 2026 15:00:30 -0600
Subject: [PATCH] feat: keep client changes to a `Warp` separate from the
 values being streamed in

---
 documentation/docs/06-runtime/05-warp.md      |   2 +
 .../svelte/src/internal/client/types.d.ts     |   5 +
 packages/svelte/src/internal/client/warp.js   | 245 ++++++++++++++++--
 .../svelte/src/internal/client/warp.test.ts   | 158 +++++++++++
 .../svelte/src/internal/server/renderer.js    |  46 ++--
 .../svelte/src/internal/server/warp.test.ts   |   6 +-
 packages/svelte/types/index.d.ts              |   3 +
 7 files changed, 431 insertions(+), 34 deletions(-)
 create mode 100644 packages/svelte/src/internal/client/warp.test.ts

diff --git a/documentation/docs/06-runtime/05-warp.md b/documentation/docs/06-runtime/05-warp.md
index 14c4000b24..f23b4b39c7 100644
--- a/documentation/docs/06-runtime/05-warp.md
+++ b/documentation/docs/06-runtime/05-warp.md
@@ -159,6 +159,8 @@ response.end('</body></html>');
 
 If you stop iterating over `tail` early, the background work is stopped. Since the streamed `<script>` tags can't be known in advance, streaming can't be used with `csp: { hash: true }` — use a `nonce` instead.
 
+You can still change a `Warp` on the client while values are streaming in. Your changes take precedence over the values from the server, including ones that arrive later — for example, after `warp.clear()`, values streamed in afterwards won't be visible. This means that a component which is still waiting for data from the server will load it again on the client instead.
+
 ## CSP
 
 `Warp` adds an inline `<script>` block to the `head` returned from `render`. If you're using [Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CSP) (CSP), this script will likely fail to run. You can provide a `nonce` to `render`:
diff --git a/packages/svelte/src/internal/client/types.d.ts b/packages/svelte/src/internal/client/types.d.ts
index 2c564c6a25..2e787a10d5 100644
--- a/packages/svelte/src/internal/client/types.d.ts
+++ b/packages/svelte/src/internal/client/types.d.ts
@@ -1,6 +1,7 @@
 import type { Store, WarpKey } from '#shared';
 import { STATE_SYMBOL } from './constants.js';
 import type { Batch } from './reactivity/batch.js';
+import type { Overlay } from './warp.js';
 import type { Effect, Source, Value } from './reactivity/types.js';
 
 declare global {
@@ -8,6 +9,10 @@ declare global {
 		__svelte?: {
 			/** `Warp` values, keyed by the `Warp`'s id */
 			w?: Map<string, Map<WarpKey, unknown>>;
+			/** The number of `Warp` streams that haven't finished yet */
+			s?: number;
+			/** Changes made to `Warp`s while values were streaming in, keyed by the `Warp`'s id */
+			m?: Map<string, Overlay>;
 		};
 	}
 }
diff --git a/packages/svelte/src/internal/client/warp.js b/packages/svelte/src/internal/client/warp.js
index 7b5e8a19b9..b900bc3e7f 100644
--- a/packages/svelte/src/internal/client/warp.js
+++ b/packages/svelte/src/internal/client/warp.js
@@ -27,13 +27,18 @@ export class Warp {
 		this.#id = id;
 	}
 
-	/** @returns {Map<K, V>} */
+	/**
+	 * The values for this `Warp`. If changes were made to them while values were still streaming
+	 * in from the server, and the stream has since finished, they're applied first
+	 * @returns {Map<K, V>}
+	 */
 	#values() {
 		if (!async_mode_flag) {
 			e.experimental_async_required('Warp');
 		}
 
-		const store = ((window.__svelte ??= {}).w ??= new Map());
+		const svelte = (window.__svelte ??= {});
+		const store = (svelte.w ??= new Map());
 		let values = store.get(this.#id);
 
 		if (values === undefined) {
@@ -41,15 +46,47 @@ export class Warp {
 			store.set(this.#id, values);
 		}
 
+		const overlay = svelte.m?.get(this.#id);
+
+		if (overlay !== undefined && !is_streaming()) {
+			overlay.apply(values);
+			svelte.m?.delete(this.#id);
+		}
+
 		return /** @type {Map<K, V>} */ (values);
 	}
 
+	/**
+	 * While values are still streaming in from the server, changes are made to an overlay rather
+	 * than the values themselves, so that they can't interfere with the stream (or vice versa)
+	 * @param {boolean} create
+	 * @returns {Overlay | null}
+	 */
+	#overlay(create) {
+		if (!is_streaming()) return null;
+
+		const svelte = /** @type {NonNullable<Window['__svelte']>} */ (window.__svelte);
+		let overlay = svelte.m?.get(this.#id) ?? null;
+
+		if (overlay === null && create) {
+			overlay = new Overlay();
+			(svelte.m ??= new Map()).set(this.#id, overlay);
+		}
+
+		return overlay;
+	}
+
 	/**
 	 * @param {K} key
 	 * @returns {V | undefined}
 	 */
 	get(key) {
-		return this.#values().get(key);
+		const values = this.#values();
+		const overlay = this.#overlay(false);
+
+		return /** @type {V | undefined} */ (
+			overlay === null ? values.get(key) : overlay.get(values, key)
+		);
 	}
 
 	/**
@@ -57,7 +94,10 @@ export class Warp {
 	 * @returns {boolean}
 	 */
 	has(key) {
-		return this.#values().has(key);
+		const values = this.#values();
+		const overlay = this.#overlay(false);
+
+		return overlay === null ? values.has(key) : overlay.has(values, key);
 	}
 
 	/**
@@ -66,7 +106,15 @@ export class Warp {
 	 * @returns {this}
 	 */
 	set(key, value) {
-		this.#values().set(key, value);
+		const values = this.#values();
+		const overlay = this.#overlay(true);
+
+		if (overlay === null) {
+			values.set(key, value);
+		} else {
+			overlay.set(key, value);
+		}
+
 		return this;
 	}
 
@@ -77,7 +125,13 @@ export class Warp {
 	 * @returns {V}
 	 */
 	getOrInsert(key, value) {
-		return get_or_insert(this.#values(), key, value);
+		if (!is_streaming()) {
+			return get_or_insert(this.#values(), key, value);
+		}
+
+		if (this.has(key)) return /** @type {V} */ (this.get(key));
+		this.set(key, value);
+		return value;
 	}
 
 	/**
@@ -87,7 +141,14 @@ export class Warp {
 	 * @returns {V}
 	 */
 	getOrInsertComputed(key, callback) {
-		return get_or_insert_computed(this.#values(), key, callback);
+		if (!is_streaming()) {
+			return get_or_insert_computed(this.#values(), key, callback);
+		}
+
+		if (this.has(key)) return /** @type {V} */ (this.get(key));
+		const value = callback(key);
+		this.set(key, value);
+		return value;
 	}
 
 	/**
@@ -95,11 +156,21 @@ export class Warp {
 	 * @returns {boolean}
 	 */
 	delete(key) {
-		return this.#values().delete(key);
+		const values = this.#values();
+		const overlay = this.#overlay(true);
+
+		return overlay === null ? values.delete(key) : overlay.delete(values, key);
 	}
 
 	clear() {
-		this.#values().clear();
+		const values = this.#values();
+		const overlay = this.#overlay(true);
+
+		if (overlay === null) {
+			values.clear();
+		} else {
+			overlay.clear();
+		}
 	}
 
 	/**
@@ -107,30 +178,170 @@ export class Warp {
 	 * @param {any} [this_arg]
 	 */
 	forEach(callback, this_arg) {
-		this.#values().forEach(callback, this_arg);
+		for (const [key, value] of this.entries()) {
+			callback.call(this_arg, value, key, this);
+		}
 	}
 
+	/** @returns {IterableIterator<[K, V]>} */
 	entries() {
-		return this.#values().entries();
+		const values = this.#values();
+		const overlay = this.#overlay(false);
+
+		return overlay === null
+			? values.entries()
+			: /** @type {IterableIterator<[K, V]>} */ (overlay.entries(values));
 	}
 
-	keys() {
-		return this.#values().keys();
+	/** @returns {IterableIterator<K>} */
+	*keys() {
+		for (const [key] of this.entries()) yield key;
 	}
 
-	values() {
-		return this.#values().values();
+	/** @returns {IterableIterator<V>} */
+	*values() {
+		for (const [, value] of this.entries()) yield value;
 	}
 
 	get size() {
-		return this.#values().size;
+		const values = this.#values();
+		const overlay = this.#overlay(false);
+
+		if (overlay === null) return values.size;
+
+		let size = 0;
+		for (const _ of overlay.entries(values)) size += 1;
+		return size;
 	}
 
 	[Symbol.iterator]() {
-		return this.#values()[Symbol.iterator]();
+		return this.entries();
 	}
 
 	get [Symbol.toStringTag]() {
 		return 'Warp';
 	}
 }
+
+/**
+ * Whether values are still streaming in from the server. Streaming can only happen while the document is
+ * still loading, so if the connection drops before the stream finishes, this still becomes `false` eventually
+ */
+function is_streaming() {
+	return (window.__svelte?.s ?? 0) > 0 && document.readyState === 'loading';
+}
+
+const TOMBSTONE = Symbol('deleted');
+
+/**
+ * Changes made to a `Warp` while values are still streaming in from the server. They take precedence
+ * over the values from the server (including ones that arrive later), and are applied once the stream
+ * has finished.
+ */
+export class Overlay {
+	/** Whether the `Warp` was cleared, which hides all the values from the server */
+	cleared = false;
+
+	/**
+	 * Values that were set (or deleted, as `TOMBSTONE`) since the `Warp` was last cleared
+	 * @type {Map<WarpKey, unknown>}
+	 */
+	changes = new Map();
+
+	/**
+	 * Keys from the server that were deleted and then set again, which moves them to the end
+	 * @type {Set<WarpKey>}
+	 */
+	moved = new Set();
+
+	/**
+	 * @param {Map<WarpKey, unknown>} values
+	 * @param {WarpKey} key
+	 */
+	get(values, key) {
+		if (this.changes.has(key)) {
+			const value = this.changes.get(key);
+			return value === TOMBSTONE ? undefined : value;
+		}
+
+		return this.cleared ? undefined : values.get(key);
+	}
+
+	/**
+	 * @param {Map<WarpKey, unknown>} values
+	 * @param {WarpKey} key
+	 */
+	has(values, key) {
+		if (this.changes.has(key)) return this.changes.get(key) !== TOMBSTONE;
+		return !this.cleared && values.has(key);
+	}
+
+	/**
+	 * @param {WarpKey} key
+	 * @param {unknown} value
+	 */
+	set(key, value) {
+		if (this.changes.get(key) === TOMBSTONE) {
+			// like in a `Map`, a key that was deleted and then set again goes to the end
+			this.changes.delete(key);
+			this.moved.add(key);
+		}
+
+		this.changes.set(key, value);
+	}
+
+	/**
+	 * @param {Map<WarpKey, unknown>} values
+	 * @param {WarpKey} key
+	 */
+	delete(values, key) {
+		const existed = this.has(values, key);
+
+		// even if it doesn't exist yet, it may still arrive from the server, and must stay deleted
+		this.changes.set(key, TOMBSTONE);
+		this.moved.delete(key);
+
+		return existed;
+	}
+
+	clear() {
+		this.cleared = true;
+		this.changes.clear();
+		this.moved.clear();
+	}
+
+	/**
+	 * @param {Map<WarpKey, unknown>} values
+	 * @returns {Generator<[WarpKey, unknown]>}
+	 */
+	*entries(values) {
+		if (!this.cleared) {
+			for (const [key, value] of values) {
+				if (this.moved.has(key)) continue;
+
+				if (this.changes.has(key)) {
+					const change = this.changes.get(key);
+					if (change !== TOMBSTONE) yield [key, change];
+				} else {
+					yield [key, value];
+				}
+			}
+		}
+
+		for (const [key, value] of this.changes) {
+			if (value === TOMBSTONE) continue;
+			if (!this.cleared && !this.moved.has(key) && values.has(key)) continue;
+			yield [key, value];
+		}
+	}
+
+	/** @param {Map<WarpKey, unknown>} values */
+	apply(values) {
+		if (this.cleared) values.clear();
+
+		for (const [key, value] of this.changes) {
+			if (value === TOMBSTONE || this.moved.has(key)) values.delete(key);
+			if (value !== TOMBSTONE) values.set(key, value);
+		}
+	}
+}
diff --git a/packages/svelte/src/internal/client/warp.test.ts b/packages/svelte/src/internal/client/warp.test.ts
new file mode 100644
index 0000000000..b6115c6427
--- /dev/null
+++ b/packages/svelte/src/internal/client/warp.test.ts
@@ -0,0 +1,158 @@
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { Warp } from './warp.js';
+import { disable_async_mode_flag, enable_async_mode_flag } from '../flags/index.js';
+
+let window: {
+	__svelte?: { w?: Map<string, Map<unknown, unknown>>; s?: number; m?: Map<any, any> };
+};
+let document: { readyState: string };
+
+/** the values the stream writes to */
+function base() {
+	return window.__svelte!.w!.get('test')!;
+}
+
+beforeEach(() => {
+	enable_async_mode_flag();
+	window = {
+		__svelte: {
+			w: new Map([
+				[
+					'test',
+					new Map([
+						['a', 1],
+						['b', 2]
+					])
+				]
+			])
+		}
+	};
+	document = { readyState: 'loading' };
+	vi.stubGlobal('window', window);
+	vi.stubGlobal('document', document);
+});
+
+afterEach(() => {
+	disable_async_mode_flag();
+	vi.unstubAllGlobals();
+});
+
+describe('when not streaming', () => {
+	test('changes the values directly', () => {
+		const warp = new Warp('test');
+		warp.set('c', 3);
+		warp.delete('a');
+
+		expect([...base()]).toEqual([
+			['b', 2],
+			['c', 3]
+		]);
+	});
+});
+
+describe('while streaming', () => {
+	beforeEach(() => {
+		window.__svelte!.s = 1;
+	});
+
+	test('does not change the values the stream writes to', () => {
+		const warp = new Warp('test');
+		warp.set('a', 10);
+		warp.delete('b');
+		warp.set('c', 3);
+
+		expect([...base()]).toEqual([
+			['a', 1],
+			['b', 2]
+		]);
+
+		expect(warp.get('a')).toBe(10);
+		expect(warp.has('b')).toBe(false);
+		expect(warp.size).toBe(2);
+		expect([...warp]).toEqual([
+			['a', 10],
+			['c', 3]
+		]);
+	});
+
+	test('shows values that arrive later, unless they were changed', () => {
+		const warp = new Warp('test');
+		warp.set('late', 'client');
+		warp.delete('deleted');
+
+		base().set('late', 'server');
+		base().set('deleted', 'server');
+		base().set('new', 'server');
+
+		expect(warp.get('late')).toBe('client');
+		expect(warp.has('deleted')).toBe(false);
+		expect(warp.get('new')).toBe('server');
+	});
+
+	test('handles setting values after clearing', () => {
+		const warp = new Warp('test');
+		warp.set('c', 3);
+		warp.clear();
+		warp.set('b', 20);
+
+		// arrives after the clear, so is hidden by it
+		base().set('late', 'server');
+
+		expect(warp.has('a')).toBe(false);
+		expect(warp.get('b')).toBe(20);
+		expect(warp.has('c')).toBe(false);
+		expect(warp.has('late')).toBe(false);
+		expect([...warp.keys()]).toEqual(['b']);
+	});
+
+	test('keeps the order a Map would have', () => {
+		const warp = new Warp('test');
+		warp.set('c', 3);
+		warp.delete('a');
+		warp.set('a', 10);
+		warp.set('b', 20);
+
+		const expected = [
+			['b', 20],
+			['c', 3],
+			['a', 10]
+		];
+
+		expect([...warp]).toEqual(expected);
+
+		window.__svelte!.s = 0;
+		expect([...warp]).toEqual(expected);
+		expect([...base()]).toEqual(expected);
+	});
+
+	test('getOrInsertComputed uses the overlay', () => {
+		const warp = new Warp('test');
+		warp.delete('a');
+
+		expect(warp.getOrInsertComputed('a', () => 10)).toBe(10);
+		expect(warp.getOrInsertComputed('b', () => 20)).toBe(2);
+		expect(base().get('a')).toBe(1);
+	});
+
+	test('applies the changes once the stream finishes', () => {
+		const warp = new Warp('test');
+		warp.clear();
+		warp.set('c', 3);
+
+		window.__svelte!.s = 0;
+
+		expect([...warp]).toEqual([['c', 3]]);
+		expect([...base()]).toEqual([['c', 3]]);
+		expect(window.__svelte!.m!.size).toBe(0);
+	});
+
+	test('applies the changes once the document has loaded, even if the stream did not finish', () => {
+		const warp = new Warp('test');
+		warp.delete('a');
+
+		document.readyState = 'interactive';
+
+		expect(warp.has('a')).toBe(false);
+		expect([...base()]).toEqual([['b', 2]]);
+	});
+});
diff --git a/packages/svelte/src/internal/server/renderer.js b/packages/svelte/src/internal/server/renderer.js
index 96e3cc67fe..9a6325add4 100644
--- a/packages/svelte/src/internal/server/renderer.js
+++ b/packages/svelte/src/internal/server/renderer.js
@@ -1040,9 +1040,15 @@ export class Renderer {
 			serialization_failed(serialization_error);
 		}
 
+		const streaming = late !== null && ready.next !== null;
+
 		const body = `
 			{
-				const w = (window.__svelte ??= {}).w ??= new Map();
+				const w = (window.__svelte ??= {}).w ??= new Map();${
+					// while this is above zero, changes made to the client's `Warp`s are kept separate,
+					// so they can't interfere with the values that are still being streamed in
+					streaming ? '\n\t\t\t\twindow.__svelte.s = (window.__svelte.s ?? 0) + 1;' : ''
+				}
 
 				for (const [id, values] of ${late === null ? head : `(${head})[0]`}) {
 					const existing = w.get(id);
@@ -1068,10 +1074,14 @@ export class Renderer {
 
 		return {
 			head: `\n\t\t<script${csp_attr}>${body}</script>`,
-			tail:
-				late === null || !ready.next
-					? empty()
-					: stream_scripts(tail, ready.next, csp_attr, late.close)
+			tail: streaming
+				? stream_scripts(
+						tail,
+						/** @type {Promise<IteratorResult<string>>} */ (ready.next),
+						csp_attr,
+						late.close
+					)
+				: empty()
 		};
 	}
 
@@ -1226,26 +1236,30 @@ function stream_scripts(tail, next, csp_attr, close) {
  * @returns {AsyncGenerator<string>}
  */
 async function* generate_scripts(tail, next, csp_attr) {
-	while (true) {
-		const result = await next;
-		if (result.done) return;
+	let done = false;
 
-		let code = result.value;
-		next = tail.next();
+	while (!done) {
+		let code = '';
+		let result = await next;
 
 		while (true) {
+			if (result.done) {
+				// tell the client that this stream has finished
+				code += 'window.__svelte.s -= 1;';
+				done = true;
+				break;
+			}
+
+			code += result.value;
+			next = tail.next();
+
 			const more = await Promise.race([
 				next,
 				new Promise((fulfil) => setTimeout(() => fulfil(MACROTASK), 0))
 			]);
 
 			if (more === MACROTASK) break;
-
-			const { done, value } = /** @type {IteratorResult<string>} */ (more);
-			if (done) break;
-
-			code += value;
-			next = tail.next();
+			result = /** @type {IteratorResult<string>} */ (more);
 		}
 
 		yield `<script${csp_attr}>${code}</script>`;
diff --git a/packages/svelte/src/internal/server/warp.test.ts b/packages/svelte/src/internal/server/warp.test.ts
index f8577dbf38..98ad8aa3dd 100644
--- a/packages/svelte/src/internal/server/warp.test.ts
+++ b/packages/svelte/src/internal/server/warp.test.ts
@@ -318,7 +318,7 @@ describe('streaming', () => {
 
 	/** Collects the tail, and returns the revived values once everything has been evaluated */
 	async function revive_streamed(head: string, tail: AsyncIterable<string>) {
-		const window: { __svelte?: { w?: Map<string, Map<unknown, unknown>> } } = {};
+		const window: { __svelte?: { w?: Map<string, Map<unknown, unknown>>; s?: number } } = {};
 		const run = (html: string) => {
 			for (const [, script] of html.matchAll(/<script(?:\s[^>]*)?>([\s\S]*?)<\/script>/g)) {
 				new Function('window', script)(window);
@@ -329,10 +329,14 @@ describe('streaming', () => {
 
 		const chunks: string[] = [];
 		for await (const chunk of tail) {
+			// the client is told that a stream is in progress until the last chunk
+			expect(window.__svelte?.s).toBe(1);
 			chunks.push(chunk);
 			run(chunk);
 		}
 
+		expect(window.__svelte?.s).toBe(0);
+
 		return { values: window.__svelte?.w, chunks };
 	}
 
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index fee37e2ec2..f025691b50 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -497,8 +497,11 @@ declare module 'svelte' {
 		clear(): void;
 		
 		forEach(callback: (value: V, key: K, map: Map<K, V>) => void, this_arg?: any): void;
+		
 		entries(): IterableIterator<[K, V]>;
+		
 		keys(): IterableIterator<K>;
+		
 		values(): IterableIterator<V>;
 		get size(): number;
 		[Symbol.iterator](): IterableIterator<[K, V]>;