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fix(web): preload the root error page so hydration skips a round trip #347

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Merged
botandbang merged 1 commit into from
Nov 1, 2026
Merged

fix(web): preload the root error page so hydration skips a round trip #347

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9 changes: 5 additions & 4 deletions apps/web/tests/build.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -60,8 +60,9 @@ beforeAll(async () => {
return [...keys].map((each) => chunks[each].file);
};

// Node 0 is the root layout, which every page renders.
initialLoad = new Set([...manifest.client.imports, ...nodeScripts(0)]);
// Node 0 is the root layout, which every page renders, and node 1 the root error page, which the
// client loads on every page before it hydrates.
initialLoad = new Set([...manifest.client.imports, ...nodeScripts(0), ...nodeScripts(1)]);

routes = manifest.routes.flatMap(({ id, page }) => {
if (!page) return [];
Expand Down Expand Up @@ -92,8 +93,8 @@ it('ships no sourcemaps to the browser', () => {
// that needs more than that is worth deciding on, not finding here later.

it('keeps what every page loads within its budget', () => {
// The entry and the root layout with everything they import: SvelteKit's and Svelte's runtimes,
// devalue, and the small components pages share (vite.config.ts). About 172 KiB.
// The entry, the root layout and the root error page with everything they import: SvelteKit's and
// Svelte's runtimes, devalue, and the small components pages share (vite.config.ts). About 179 KiB.
expect(size(initialLoad)).toBeLessThan(180 * 1024);
});

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15 changes: 15 additions & 0 deletions patches/@sveltejs__kit@3.1.0.patch
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
diff --git a/src/runtime/server/page/render.js b/src/runtime/server/page/render.js
index 0a8042eb03fa01a378517d8b5ef48891438122be..6106bfd9b62b535f1de568ab913302c530f6d1d6 100644
--- a/src/runtime/server/page/render.js
+++ b/src/runtime/server/page/render.js
@@ -367,6 +367,10 @@ export async function render_response({
}

if (!client.inline) {
+ // The client awaits the root error page before it hydrates (`start` in client.js), so preload
+ // its JS too, or the browser only finds it after start.js has run (sveltejs/kit#17415).
+ for (const url of (await manifest.nodes[1]()).imports) modulepreloads.add(url);
+
for (const dep of modulepreloads) {
const path = prefixed(dep);

11 changes: 6 additions & 5 deletions pnpm-lock.yaml
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5 changes: 5 additions & 0 deletions pnpm-workspace.yaml
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Expand Up @@ -69,6 +69,11 @@ overrides:
# release has the fix (#6214).
patchedDependencies:
'@stryker-mutator/vitest-runner@10.0.0': patches/@stryker-mutator__vitest-runner@10.0.0.patch
# SvelteKit's client imports the root error page before it hydrates, but the server doesn't
# preload it, so every page fetched it a round trip late (sveltejs/kit#17415). That round trip put
# /projects/halflight's simulated LCP over Lighthouse's budget on a busy machine. Drop the patch
# once a release has the fix (sveltejs/kit#17416).
'@sveltejs/kit@3.1.0': patches/@sveltejs__kit@3.1.0.patch
# zod 3.x reaches prod only through @atproto/* (^3.23.8), and no 3.x release has the fix (4.6.6
# does). Drop this once @atproto moves to zod 4.
auditConfig:
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