From 477b2aed124158d5dca9704635b0156278c8a440 Mon Sep 17 00:00:00 2001
From: Ruben Fiszel <ruben@windmill.dev>
Date: Sat, 26 Sep 2026 23:05:04 +0200
Subject: [PATCH] perf: keep monaco and deferred shell chunks off every page's
 load

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
---
 .../copilot/chat/AiChatLayout.svelte          |  3 +-
 .../src/lib/components/home/ItemsList.svelte  | 21 ++++++---
 .../lib/components/search/RunsSearch.svelte   | 12 +++--
 frontend/src/lib/utils/paint.ts               | 47 +++++++++++++++++++
 .../src/routes/(root)/(logged)/+layout.svelte | 45 ++++++++++++++----
 frontend/src/routes/(root)/+layout.svelte     | 14 +++++-
 frontend/vite.config.js                       | 22 ++++++---
 7 files changed, 136 insertions(+), 28 deletions(-)

diff --git a/frontend/src/lib/components/copilot/chat/AiChatLayout.svelte b/frontend/src/lib/components/copilot/chat/AiChatLayout.svelte
index edcf9808cd..c85ef4189f 100644
--- a/frontend/src/lib/components/copilot/chat/AiChatLayout.svelte
+++ b/frontend/src/lib/components/copilot/chat/AiChatLayout.svelte
@@ -11,6 +11,7 @@
 	import { onDestroy } from 'svelte'
 	import Button from '$lib/components/common/button/Button.svelte'
 	import { Menu } from 'lucide-svelte'
+	import { whenPageIdle } from '$lib/utils/paint'
 
 	interface Props {
 		noPadding?: boolean
@@ -102,7 +103,7 @@
 {#if !disableAi}
 	<!-- Dynamic imports here and for AIChat: this layout wraps every workspace page, so its
 	     static imports gate first paint. -->
-	{#await import('./CreatedResourceActionDrawers.svelte') then CreatedResourceActionDrawers}
+	{#await whenPageIdle().then(() => import('./CreatedResourceActionDrawers.svelte')) then CreatedResourceActionDrawers}
 		<CreatedResourceActionDrawers.default />
 	{/await}
 	<Splitpanes horizontal={false} class="flex-1 min-h-0">
diff --git a/frontend/src/lib/components/home/ItemsList.svelte b/frontend/src/lib/components/home/ItemsList.svelte
index 45c30005a2..dc7d406349 100644
--- a/frontend/src/lib/components/home/ItemsList.svelte
+++ b/frontend/src/lib/components/home/ItemsList.svelte
@@ -1009,6 +1009,12 @@
 	// would put two of them on the page at once while the workspace is still empty.
 	let hubPick = $state<HubProjectPick | undefined>(undefined)
 	let hubPickerOpen = $state(false)
+	// Mounted on the first pick and kept, so its dismissal and in-flight run outlive `hubPick`.
+	let importModalMounted = $state(false)
+	function pickHubProject(project: HubProjectPick) {
+		importModalMounted = true
+		hubPick = project
+	}
 
 	/**
 	 * Whether a workspace the default listing found empty is empty at all, or just has nothing
@@ -1941,7 +1947,7 @@
 						<WorkspaceEmptyState
 							archivedOnly={archivedProbe?.hasArchived === true}
 							canCreate={canCreateHere}
-							onPick={(project) => (hubPick = project)}
+							onPick={pickHubProject}
 							onShowArchived={() => (filterValues.val = { ...filterValues.val, archived: true })}
 						/>
 					{:else}
@@ -2073,13 +2079,16 @@
 	onClose={() => (hubPickerOpen = false)}
 	onPick={(project) => {
 		hubPickerOpen = false
-		hubPick = project
+		pickHubProject(project)
 	}}
 />
-<!-- Dynamic: the import flow statically reaches ~400 modules (resource setup, SQL editor). -->
-{#await import('./ImportProjectModal.svelte') then ImportProjectModal}
-	<ImportProjectModal.default pick={hubPick} onClose={() => (hubPick = undefined)} {onImported} />
-{/await}
+<!-- Dynamic and on demand: the import flow statically reaches ~400 modules, Monaco among
+     them (SQL editor). -->
+{#if importModalMounted}
+	{#await import('./ImportProjectModal.svelte') then ImportProjectModal}
+		<ImportProjectModal.default pick={hubPick} onClose={() => (hubPick = undefined)} {onImported} />
+	{/await}
+{/if}
 
 <style>
 	/* Rows arriving after an import, one after another. The animation is declared on the
diff --git a/frontend/src/lib/components/search/RunsSearch.svelte b/frontend/src/lib/components/search/RunsSearch.svelte
index 3eedfb297c..326884a670 100644
--- a/frontend/src/lib/components/search/RunsSearch.svelte
+++ b/frontend/src/lib/components/search/RunsSearch.svelte
@@ -9,7 +9,6 @@
 	import QuickMenuItem from './QuickMenuItem.svelte'
 	import { goto } from '$app/navigation'
 	import { displayDateOnly } from '$lib/utils'
-	import JobPreview from '../runs/JobRunsPreview.svelte'
 
 	let debounceTimeout: any = undefined
 	const debouncePeriod: number = 1000
@@ -210,7 +209,11 @@
 				Select a result to preview
 			{:else}
 				<div class="h-[95%] overflow-y-scroll">
-					<JobPreview id={selectedItem?.document?.id[0]} workspace={selectedWorkspace} />
+					<!-- Dynamic: the global search modal is warmed on every page, and the preview
+					     reaches Monaco through the flow graph. -->
+					{#await import('../runs/JobRunsPreview.svelte') then JobPreview}
+						<JobPreview.default id={selectedItem?.document?.id[0]} workspace={selectedWorkspace} />
+					{/await}
 				</div>
 			{/if}
 			<div class="flex flex-row pt-3 pl-4 items-center text-xs text-secondary">
@@ -246,8 +249,9 @@
 				<div class="text-sm">
 					Note that new runs might take a while to become searchable (by default ~5min).
 					{#if indexMetadata?.max_index_time_window_secs}
-						Search only covers the last {Math.round(indexMetadata.max_index_time_window_secs / 86400)} day(s),
-						configurable in instance settings under Indexer.
+						Search only covers the last {Math.round(
+							indexMetadata.max_index_time_window_secs / 86400
+						)} day(s), configurable in instance settings under Indexer.
 					{/if}
 				</div>
 				{#if !$enterpriseLicense}
diff --git a/frontend/src/lib/utils/paint.ts b/frontend/src/lib/utils/paint.ts
index e17f20fbfb..cad37328c3 100644
--- a/frontend/src/lib/utils/paint.ts
+++ b/frontend/src/lib/utils/paint.ts
@@ -30,3 +30,50 @@ export async function tickPainted(): Promise<void> {
 	await nextAnimationFrame()
 	await nextAnimationFrame()
 }
+
+const QUIET_MS = 300
+const MAX_WAIT_MS = 5000
+let pageIdle: Promise<void> | undefined
+
+/**
+ * Resolve once the first page has loaded and its network has gone quiet (no resource
+ * finished for `QUIET_MS`, capped at `MAX_WAIT_MS` after `load`), then the main thread
+ * is idle. Gate prefetches of chunks the page does not need yet on it: started any
+ * earlier they compete with the page's own chunks and API calls for connections and
+ * bandwidth, which delays its first content. Resolves once per tab.
+ */
+export function whenPageIdle(): Promise<void> {
+	return (pageIdle ??= new Promise((resolve) => {
+		if (typeof window === 'undefined') return resolve()
+		const settle = () => {
+			const loadedAt = performance.now()
+			let timer: ReturnType<typeof setTimeout> | undefined
+			let observer: PerformanceObserver | undefined
+			let finished = false
+			const done = () => {
+				if (finished) return
+				finished = true
+				observer?.disconnect()
+				clearTimeout(timer)
+				if (typeof requestIdleCallback === 'function') {
+					requestIdleCallback(() => resolve(), { timeout: 1000 })
+				} else {
+					resolve()
+				}
+			}
+			const arm = () => {
+				if (finished) return
+				clearTimeout(timer)
+				const left = MAX_WAIT_MS - (performance.now() - loadedAt)
+				timer = setTimeout(done, Math.max(0, Math.min(QUIET_MS, left)))
+			}
+			try {
+				observer = new PerformanceObserver(arm)
+				observer.observe({ type: 'resource' })
+			} catch {}
+			arm()
+		}
+		if (document.readyState === 'complete') settle()
+		else window.addEventListener('load', settle, { once: true })
+	}))
+}
diff --git a/frontend/src/routes/(root)/(logged)/+layout.svelte b/frontend/src/routes/(root)/(logged)/+layout.svelte
index 5bf053d6f2..376a246a3a 100644
--- a/frontend/src/routes/(root)/(logged)/+layout.svelte
+++ b/frontend/src/routes/(root)/(logged)/+layout.svelte
@@ -65,6 +65,7 @@
 		getFavoriteLabel
 	} from '$lib/components/sidebar/FavoriteMenu.svelte'
 	import { SUPERADMIN_SETTINGS_HASH, USER_SETTINGS_HASH } from '$lib/components/sidebar/settings'
+	import { whenPageIdle } from '$lib/utils/paint'
 	import { isCloudHosted } from '$lib/cloud'
 	import { PanelLeftClose, PanelLeftOpen, Home, Play, Search, WandSparkles } from 'lucide-svelte'
 	import { getUserExt } from '$lib/user'
@@ -254,6 +255,20 @@
 	})
 	let userSettings: UserSettings | undefined = $state()
 	let superadminSettings: SuperadminSettings | undefined = $state()
+
+	// The shell's drawers and modals mount on first request, or once the page is idle so
+	// they open instantly. Not before: their chunks would compete with the page's own.
+	// Superadmin settings only mounts on request, as it reaches Monaco.
+	let pageIdle = $state(false)
+	whenPageIdle().then(() => (pageIdle = true))
+	let userSettingsRequested = $state(false)
+	let superadminSettingsRequested = $state(false)
+	let searchModalRequested = $state(false)
+	$effect(() => {
+		const hash = page.url.hash
+		if (hash.startsWith(USER_SETTINGS_HASH)) userSettingsRequested = true
+		if (hash === SUPERADMIN_SETTINGS_HASH) superadminSettingsRequested = true
+	})
 	let menuHidden = $state(false)
 	let isDarkMode = useIsDarkMode()
 	let darkMode = $derived(isDarkMode.val)
@@ -695,7 +710,10 @@
 
 	function openSearchModal(text?: string, stack?: OverlayStack): void {
 		if (globalSearchModal) globalSearchModal.openSearchWithPrefilledText(text, stack)
-		else if (searchModalMounts) pendingSearchOpen = { text, stack }
+		else if (searchModalMounts) {
+			pendingSearchOpen = { text, stack }
+			searchModalRequested = true
+		}
 	}
 
 	$effect(() => {
@@ -932,6 +950,11 @@
 	})
 
 	globalDbManagerDrawer.val = useDbManagerUriState()
+	// Mounted on first open and kept for its close transition: it reaches Monaco.
+	let dbManagerRequested = $state(false)
+	$effect(() => {
+		if (globalDbManagerDrawer.val?.open) dbManagerRequested = true
+	})
 
 	let globalS3FilePicker: S3FilePicker | undefined = $state()
 	$effect(() => {
@@ -1018,9 +1041,11 @@
 
 <!-- The drawers and modals below are dynamic imports: this layout wraps every workspace
      page, so whatever it imports statically gates first paint. -->
-{#await import('$lib/components/UserSettings.svelte') then UserSettings}
-	<UserSettings.default bind:this={userSettings} showMcpMode={true} />
-{/await}
+{#if pageIdle || userSettingsRequested}
+	{#await import('$lib/components/UserSettings.svelte') then UserSettings}
+		<UserSettings.default bind:this={userSettings} showMcpMode={true} />
+	{/await}
+{/if}
 {#if accountSetup.pending}
 	<FinishAccountSetup
 		bind:open={accountSetup.open}
@@ -1032,10 +1057,12 @@
 {#if page.status == 404}
 	<CenteredModal title="Page not found, redirecting you to login" loading={true}></CenteredModal>
 {:else if $userStore}
-	{#await import('$lib/components/search/GlobalSearchModal.svelte') then GlobalSearchModal}
-		<GlobalSearchModal.default bind:this={globalSearchModal} />
-	{/await}
-	{#if $superadmin}
+	{#if pageIdle || searchModalRequested}
+		{#await import('$lib/components/search/GlobalSearchModal.svelte') then GlobalSearchModal}
+			<GlobalSearchModal.default bind:this={globalSearchModal} />
+		{/await}
+	{/if}
+	{#if $superadmin && superadminSettingsRequested}
 		{#await import('$lib/components/SuperadminSettings.svelte') then SuperadminSettings}
 			<SuperadminSettings.default bind:this={superadminSettings} />
 		{/await}
@@ -1536,7 +1563,7 @@
 	<CenteredModal title="Loading user..." loading={true}></CenteredModal>
 {/if}
 
-{#if $workspaceStore && globalDbManagerDrawer.val}
+{#if $workspaceStore && globalDbManagerDrawer.val && dbManagerRequested}
 	{#await import('$lib/components/DBManagerDrawer.svelte') then DBManagerDrawer}
 		<DBManagerDrawer.default uriState={globalDbManagerDrawer.val} />
 	{/await}
diff --git a/frontend/src/routes/(root)/+layout.svelte b/frontend/src/routes/(root)/+layout.svelte
index 27a6ed20ac..e28d7bbe48 100644
--- a/frontend/src/routes/(root)/+layout.svelte
+++ b/frontend/src/routes/(root)/+layout.svelte
@@ -205,10 +205,22 @@
 					location.reload()
 					return
 				}
+				// A chunk gone from the server, typically a tab left open across an upgrade:
+				// a component loaded on demand would otherwise silently never appear.
+				if (message.startsWith('Failed to fetch dynamically imported')) {
+					console.warn(message)
+					sendUserToast(
+						'Part of the page failed to load, Windmill may have been updated',
+						true,
+						[{ label: 'Reload', callback: () => location.reload() }],
+						undefined,
+						15000
+					)
+					return
+				}
 				// Unhandled errors from Monaco Editor don't logout the user
 				if (
 					monacoEditorUnhandledErrors.includes(message) ||
-					message.startsWith('Failed to fetch dynamically imported') ||
 					message.startsWith('Unable to figure out browser width and height') ||
 					message.startsWith('Unable to read file') ||
 					message.startsWith('Could not find source file')
diff --git a/frontend/vite.config.js b/frontend/vite.config.js
index 75f7ea5dcf..d24c47c511 100644
--- a/frontend/vite.config.js
+++ b/frontend/vite.config.js
@@ -83,21 +83,29 @@ function assertAcyclicChunks() {
  * These pages also serve raw apps, which only need a small shell around their bundle's
  * iframe. One static import of AppPreview (or of anything reaching monaco) makes every
  * public app page preload ~650 chunks instead of ~55. See loadAppPreview.ts.
+ *
+ * The workspace shell and home page must not reach monaco either: it is ~2 MB gzipped
+ * that every page would download whether or not it shows an editor.
  */
 function assertLeanPublicAppRoutes() {
+	const monaco = '/node_modules/monaco-editor/'
 	// Route directories, so +page.js counts as well as +page.svelte.
-	const routes = ['/src/routes/public/[workspace]/[...secret]/', '/src/routes/a/[...path]/']
-	const forbidden = [
-		'/src/lib/components/apps/editor/AppPreview.svelte',
-		'/node_modules/monaco-editor/'
-	]
+	const forbiddenByRoute = {
+		'/src/routes/public/[workspace]/[...secret]/': [
+			'/src/lib/components/apps/editor/AppPreview.svelte',
+			monaco
+		],
+		'/src/routes/a/[...path]/': ['/src/lib/components/apps/editor/AppPreview.svelte', monaco],
+		'/src/routes/(root)/(logged)/+layout.svelte': [monaco],
+		'/src/routes/(root)/(logged)/+page.svelte': [monaco]
+	}
 	return {
 		name: 'wm-assert-lean-public-app-routes',
 		generateBundle(_options, bundle) {
 			const chunks = Object.entries(bundle).filter(([, c]) => c.type === 'chunk')
 			if (!chunks.some(([file]) => file.startsWith('_app/immutable/'))) return
 			const idsOf = (c) => c.moduleIds ?? Object.keys(c.modules ?? {})
-			for (const route of routes) {
+			for (const [route, forbidden] of Object.entries(forbiddenByRoute)) {
 				const starts = chunks
 					.filter(([, c]) => idsOf(c).some((id) => id.includes(route)))
 					.map(([file]) => file)
@@ -110,7 +118,7 @@ function assertLeanPublicAppRoutes() {
 					if (!chunk) continue
 					const hit = idsOf(chunk).find((id) => forbidden.some((f) => id.includes(f)))
 					if (hit) {
-						this.error(`${route} statically loads ${hit}; import it lazily (see loadAppPreview.ts)`)
+						this.error(`${route} statically loads ${hit}; import it lazily`)
 					}
 					for (const dep of chunk.imports ?? []) {
 						if (seen.has(dep)) continue