feat: compiled-style thumbnails and Less compilation in a worker

- thumbnails: compile the project stylesheet once per rebuild and feed it
  into the layout sandbox, compose each tile like the runtime does
  (<section data-ps-slide> around the slide), and show example HTML on
  the global tile instead of another real slide's content.
- worker: less compiles off the main thread in less-worker; a side-effect
  module shims window/document and disables the browser build's page
  auto-init, and compilation falls back inline if the worker can't start.
- code-to-element: wrap styles[] entries in <style> blocks - raw CSS text
  in the iframe head fell into <body> and rendered as literal garbage.

src/code-to-element.ts

@@ -3,6 +3,7 @@ import { domToSvgDataUrl } from './dom-to-image'
 export interface CodeToElementOptions {
     title?: string
     allowScripts?: boolean
+    /** Plain CSS strings — each becomes a `<style>` block in the iframe head. */
     styles?: string[]
 }
 
@@ -18,7 +19,8 @@ export function codeToElement(code: string, options: CodeToElementOptions = {}):
 
     iframe.title = options.title ?? 'Rendered code'
     iframe.setAttribute('sandbox', Array.from(sandbox).join(' '))
-    iframe.srcdoc = `<!doctype html><html><head><meta charset="UTF-8"><style>html,body{margin:0;min-height:100%;}</style>${(options.styles ?? []).join('\n')}</head><body>${code}</body></html>`
+    const headStyles = (options.styles ?? []).map((css) => `<style>${css}</style>`).join('\n')
+    iframe.srcdoc = `<!doctype html><html><head><meta charset="UTF-8"><style>html,body{margin:0;min-height:100%;}</style>${headStyles}</head><body>${code}</body></html>`
 
     return iframe
 }

src/main.ts

@@ -1,7 +1,13 @@
 import './style.css'
 import { createEditor } from './editor'
 import { createCodeThumbnail } from './code-to-element'
-import { compilePresentation, extractStyles } from './slides/compile'
+import {
+    compilePresentation,
+    extractStyles,
+    slideId,
+    type CompiledPresentation
+} from './slides/compile'
+import { CSS_PLACEHOLDER, HTML_PLACEHOLDER } from './slides/placeholders'
 import { createRuntime, type Runtime } from './slides/runtime'
 
 interface Slide {
@@ -51,6 +57,46 @@ const STARTER_STYLE = `// Project styles. This is ordinary Less: variables, nest
 }
 `
 
+// Shown in the global-slide tile while the global slide is empty: an example
+// frame around a sample slide, so the tile explains what the global slide is
+// for instead of rendering a blank box. Template mode mounts this same sample
+// in the author's own frame.
+const EXAMPLE_SECTION = `<section data-ps-slide="${slideId(0)}" data-ps-state="current"><h1>Example slide</h1><p>The global slide wraps every slide like this.</p></section>`
+
+const EXAMPLE_FRAME = `<style>
+    .ps-example-frame {
+        position: absolute;
+        inset: 0;
+        display: flex;
+        flex-direction: column;
+        gap: 1rem;
+        padding: 2rem;
+        background: #161a22;
+        color: #dbe4f0;
+        font-family: system-ui, sans-serif;
+    }
+    .ps-example-frame header {
+        font-size: 1.75rem;
+        font-weight: 650;
+        opacity: 0.85;
+    }
+    .ps-example-frame .ps-example-viewport {
+        position: relative;
+        flex: 1;
+        border-radius: 0.75rem;
+        overflow: hidden;
+    }
+    .ps-example-frame footer {
+        font-size: 1.25rem;
+        opacity: 0.55;
+    }
+</style>
+<div class="ps-example-frame">
+    <header>Example deck</header>
+    <div class="ps-example-viewport">${EXAMPLE_SECTION}</div>
+    <footer>Global slide — wraps every slide</footer>
+</div>`
+
 let globalSlide = ''
 
 const slides: Slide[] = [
@@ -71,6 +117,12 @@ let globalSlideElement: HTMLElement | null = null
 const slideElements = new Map<Slide, HTMLElement>()
 const thumbnailRequests = new Map<number, number>()
 let thumbnailDebounceTimer: number | undefined
+// The last good compile: thumbnails read the stylesheet from here, so a tile
+// is rendered exactly as the presentation renders it.
+let lastResult: CompiledPresentation | null = null
+// Bumped by every full list rebuild; async thumbnail builders check it so a
+// stale build can never commit over a newer one.
+let listGeneration = 0
 
 // The compiled stylesheet is the runtime's business: it lands at the global
 // slide's `<?= css ?>`, or in a `data-ps-slides` sheet in the head when the
@@ -157,6 +209,15 @@ async function rebuildPreview(): Promise<void> {
         if (currentSlide >= 0) {
             runtime.goTo(currentSlide)
         }
+
+        // Tiles embed the stylesheet (and, in template mode, the frame), so a
+        // change to either makes every tile stale. The first compile builds
+        // the list itself; render() only builds it when that compile failed.
+        const previous = lastResult
+        lastResult = result
+        if (!previous || previous.css !== result.css || previous.template !== result.template) {
+            void setSlideList()
+        }
     } catch (error) {
         // Half-typed Less should not blank the preview: keep the last good
         // output on screen and report what broke.
@@ -164,6 +225,44 @@ async function rebuildPreview(): Promise<void> {
     }
 }
 
+/**
+ * Compose a tile exactly as the runtime composes the real thing: the slide in
+ * its `<section data-ps-slide>` with state `current` (so the compiled state
+ * rules show the settled look), inside the global template when there is one,
+ * with the shared markup prepended when there is not. The global tile mounts
+ * an example slide in the frame instead — it must not show another real
+ * slide's content, which would go stale whenever that slide is edited.
+ */
+function buildThumbnailHtml(index: number, code: string): string {
+    const global = extractStyles(globalSlide).html
+    const isTemplate = HTML_PLACEHOLDER.test(global)
+
+    if (index === -1) {
+        if (!global.trim()) {
+            return EXAMPLE_FRAME
+        }
+        if (isTemplate) {
+            return global
+                .split(HTML_PLACEHOLDER)
+                .join(EXAMPLE_SECTION)
+                .split(CSS_PLACEHOLDER)
+                .join('')
+        }
+        return global
+    }
+
+    const extracted = extractStyles(code).html
+    let inner = extracted
+    if (!isTemplate && global) {
+        inner = `${global}\n${extracted}`
+    }
+    const section = `<section data-ps-slide="${slideId(index)}" data-ps-state="current">${inner}</section>`
+    if (isTemplate) {
+        return global.split(HTML_PLACEHOLDER).join(section).split(CSS_PLACEHOLDER).join('')
+    }
+    return section
+}
+
 async function generateSlideThumbnail(slide: Slide, i: number) {
     const slideElement = document.createElement('div')
     slideElement.classList.add('slide-list-item')
@@ -172,7 +271,6 @@ async function generateSlideThumbnail(slide: Slide, i: number) {
         slideElement.classList.add('global-slide')
     } else {
         slideElement.draggable = true
-        slideElements.set(slide, slideElement)
     }
 
     const slideInList = document.createElement('button')
@@ -187,9 +285,11 @@ async function generateSlideThumbnail(slide: Slide, i: number) {
     slideNumber.classList.add('slide-number')
     slideNumber.textContent = i === -1 ? 'G' : `${i + 1}`
 
-    // Style tags are the project stylesheet, never slide content: the
+    // The tile mirrors the presentation: composed markup, plus the compiled
-    // thumbnail renders markup only, exactly like the presentation does.
+    // stylesheet injected into the thumbnail document's head.
-    const thumbnail = await createCodeThumbnail(extractStyles(slide.code).html)
+    const thumbnail = await createCodeThumbnail(buildThumbnailHtml(i, slide.code), {
+        styles: lastResult?.css ? [lastResult.css] : []
+    })
     slideInList.append(slideNumber, thumbnail)
     slideElement.append(slideInList)
 
@@ -205,10 +305,6 @@ async function generateSlideThumbnail(slide: Slide, i: number) {
         slideElement.append(deleteButton)
     }
 
-    if (i === -1) {
-        globalSlideElement = slideElement
-    }
-
     if (i === currentSlide) {
         slideElement.classList.add('selected')
     }
@@ -219,9 +315,12 @@ async function generateSlideThumbnail(slide: Slide, i: number) {
 async function refreshThumbnail(index: number, slide: Slide) {
     const request = (thumbnailRequests.get(index) ?? 0) + 1
     thumbnailRequests.set(index, request)
+    const generation = listGeneration
     const oldSlideElement = index === -1 ? globalSlideElement : slideElements.get(slide)
     const slideElement = await generateSlideThumbnail(slide, index)
-    if (thumbnailRequests.get(index) !== request) {
+    // A newer refresh or a full rebuild superseded this one while it rendered;
+    // a stale tile must never replace a fresh one.
+    if (generation !== listGeneration || thumbnailRequests.get(index) !== request) {
         return
     }
 
@@ -257,15 +356,31 @@ async function mapWithConcurrency<T, R>(
 }
 
 async function setSlideList() {
+    const generation = ++listGeneration
     const allSlides = [{ code: globalSlide }, ...slides]
-    slideList.replaceChildren()
-    slideElements.clear()
-    globalSlideElement = null
-
     const elements = await mapWithConcurrency(allSlides, THUMBNAIL_CONCURRENCY, (slide, index) =>
         generateSlideThumbnail(slide, index - 1)
     )
-    slideList.append(...elements)
+    // Commit atomically: the old list stays interactive while tiles render.
+    const deckUnchanged =
+        allSlides.length === slides.length + 1 &&
+        allSlides[0].code === globalSlide &&
+        allSlides.slice(1).every((slide, offset) => slide === slides[offset])
+    if (generation !== listGeneration) {
+        return
+    }
+    if (!deckUnchanged) {
+        // A slide was added, removed or moved mid-render; this snapshot is
+        // stale, and no other rebuild is coming — start a fresh one.
+        void setSlideList()
+        return
+    }
+    slideList.replaceChildren(...elements)
+    slideElements.clear()
+    globalSlideElement = elements[0] ?? null
+    elements.slice(1).forEach((element, offset) => {
+        slideElements.set(allSlides[offset + 1], element)
+    })
     updateSelection()
 }
 
@@ -340,11 +455,18 @@ function deleteSlide(index: number) {
 addSlideButton.addEventListener('click', async () => {
     const slide = { code: '' }
     slides.push(slide)
-    const slideElement = await generateSlideThumbnail(slide, slides.length - 1)
-    slideList.append(slideElement)
-    updateSlideOrder()
     setSlide(slides.length - 1)
     scheduleCompile()
+
+    const generation = listGeneration
+    const slideElement = await generateSlideThumbnail(slide, slides.length - 1)
+    // A full rebuild started meanwhile; its snapshot already has this slide.
+    if (generation !== listGeneration) {
+        return
+    }
+    slideElements.set(slide, slideElement)
+    slideList.append(slideElement)
+    updateSlideOrder()
 })
 
 presentButton.addEventListener('click', () => {
@@ -420,9 +542,13 @@ editor.onDidChangeModelContent(() => {
 })
 
 async function render() {
-    await setSlideList()
     setSlide(currentSlide)
     await rebuildPreview()
+    // A successful first compile builds the list itself; this is only the
+    // fallback when even the boot compile failed.
+    if (!lastResult) {
+        await setSlideList()
+    }
 }
 
 render()

src/slides/compile.ts

@@ -1,11 +1,107 @@
-import less from 'less'
+import LessWorker from './less-worker?worker'
 import { CSS_PLACEHOLDER, HTML_PLACEHOLDER } from './placeholders'
 
+/**
+ * Less runs in a worker so a heavy stylesheet never blocks the editor. The
+ * worker is an optimisation, not a requirement: if it fails to start or dies
+ * mid-render, everything still in flight (and every later render) falls back
+ * to an on-demand `import('less')` on the main thread — which also keeps the
+ * big Less bundle out of the initial page load.
+ */
+interface RenderTask {
+    source: string
+    resolve: (css: string) => void
+    reject: (error: Error) => void
+}
+
+/** Reply the worker posts back: either the CSS or the failure message. */
+interface RenderReply {
+    id: number
+    css?: string
+    error?: string
+}
+
+const renderTasks = new Map<number, RenderTask>()
+let nextRenderTask = 0
+/** `undefined` until first use, then the worker — or `null` once degraded. */
+let lessWorker: Worker | null | undefined
+
+function renderInline(source: string): Promise<string> {
+    // The same auto-init as in the worker: pre-set `onReady: false` (the init
+    // copies `window.less` before its own defaults) so importing less does
+    // not hide `<body>` or rescan the app's stylesheets just because we only
+    // wanted its compiler.
+    if (typeof window !== 'undefined' && !('less' in window)) {
+        ;(window as unknown as { less?: unknown }).less = { onReady: false }
+    }
+    return import('less').then(async (module) => {
+        const { css } = await module.default.render(source)
+        return css
+    })
+}
+
+/** The worker is gone: re-run everything pending on the main thread. */
+function degradeWorker(): void {
+    if (!lessWorker) {
+        return
+    }
+    lessWorker.terminate()
+    lessWorker = null
+    const pending = Array.from(renderTasks.values())
+    renderTasks.clear()
+    for (const task of pending) {
+        renderInline(task.source).then(task.resolve).catch(task.reject)
+    }
+}
+
+function getLessWorker(): Worker | null {
+    if (lessWorker !== undefined) {
+        return lessWorker
+    }
+    try {
+        const worker = new LessWorker()
+        worker.addEventListener('message', (event: MessageEvent<RenderReply>) => {
+            const task = renderTasks.get(event.data.id)
+            if (!task) {
+                return
+            }
+            renderTasks.delete(event.data.id)
+            if (event.data.error !== undefined) {
+                task.reject(new Error(event.data.error))
+            } else {
+                task.resolve(event.data.css ?? '')
+            }
+        })
+        // Uncaught worker failure (bundle load, environment): drop to inline.
+        worker.addEventListener('error', (event) => {
+            event.preventDefault()
+            degradeWorker()
+        })
+        lessWorker = worker
+    } catch {
+        lessWorker = null
+    }
+    return lessWorker
+}
+
+function renderLess(source: string): Promise<string> {
+    const worker = getLessWorker()
+    if (!worker) {
+        return renderInline(source)
+    }
+    return new Promise<string>((resolve, reject) => {
+        const id = ++nextRenderTask
+        renderTasks.set(id, { source, resolve, reject })
+        // oxlint-disable-next-line unicorn/require-post-message-target-origin
+        worker.postMessage({ id, source })
+    })
+}
+
 /**
  * A slide's stable id. Deterministic for a given position, so recompiling the
  * project (on every keystroke) never renames a slide underneath the runtime.
  */
-function slideId(index: number): string {
+export function slideId(index: number): string {
     return `slide-${index}`
 }
 
@@ -145,15 +241,14 @@ async function compileStyles(source: string, ids: string[]): Promise<string> {
     // Fast path: without `:thisSlide` every slide resolves to the same CSS, so
     // compile once instead of once per slide.
     if (!source.includes(THIS_SLIDE)) {
-        const { css } = await less.render(source)
+        return renderLess(source)
-        return css
     }
 
     const perSlide = await Promise.all(
         ids.map(async (id) => {
             const selector = slideSelector(id)
             const scoped = source.split(THIS_SLIDE).join(selector)
-            const { css } = await less.render(scoped)
+            const css = await renderLess(scoped)
             return `/* ${selector} */\n${css}`
         })
     )

src/slides/less-worker.ts

@@ -0,0 +1,43 @@
+// Worker `postMessage` takes no targetOrigin; that rule only targets windows.
+/* oxlint-disable unicorn/require-post-message-target-origin */
+
+// Side-effect import first: it shims `window`/`document` for less' auto-init,
+// which reads them while the less module evaluates — a static import would
+// hoist above any code in this file, hence the separate module.
+import './worker-globals'
+import less from 'less'
+
+/**
+ * Renders one Less source per message so the main thread never blocks on a
+ * big stylesheet. Replies carry the task id back with either the CSS or the
+ * error message — a Less syntax error is an ordinary reply, not a crash.
+ */
+interface RenderRequest {
+    id: number
+    source: string
+}
+
+interface RenderReply {
+    id: number
+    css?: string
+    error?: string
+}
+
+// Typed narrowly on purpose: this file runs as a worker, but the program is
+// typed with the DOM libs (no WebWorker lib), so `self` here is the scope.
+const scope = self as unknown as {
+    addEventListener(type: 'message', listener: (event: MessageEvent<RenderRequest>) => void): void
+    postMessage(message: RenderReply): void
+}
+
+scope.addEventListener('message', (event) => {
+    const { id, source } = event.data
+    less.render(source)
+        .then(({ css }) => scope.postMessage({ id, css }))
+        .catch((error: unknown) => {
+            scope.postMessage({
+                id,
+                error: error instanceof Error ? error.message : String(error)
+            })
+        })
+})

src/slides/worker-globals.ts

@@ -0,0 +1,14 @@
+// less' browser build runs a page auto-init while its module loads: it
+// reaches for `window`, copies any `window.less` config, then — unless told
+// otherwise — hides `<body>` and rescans the document's stylesheets. A worker
+// has none of that, so bind `window` to the worker scope, stub the single
+// `document` probe the init makes (null-safe downstream), and pre-set
+// `onReady: false` so init stops right after building the compiler.
+// The worker imports this for side effects before `less`, so ESM evaluation
+// order guarantees the shim exists by the time the auto-init runs.
+if (typeof window === 'undefined') {
+    const worker = globalThis as unknown as Record<string, unknown>
+    worker.window = worker
+    worker.document = { getElementsByTagName: () => [] }
+    worker.less = { onReady: false }
+}