fix: scope every rule in a slide's own style block to that slide

Binding only the `:thisSlide` token still left every untokened rule in a
slide's tag compiling deck-wide, so one slide's styles kept applying to
all slides. Wrap each slide's whole style block in its section selector
instead, with `:thisSlide` resolving to `&` — Less hoists `@keyframes`/
`@font-face` out of the wrapper itself and keeps `@media` nested inside,
so only rules end up scoped. The global block stays the shared layer and
the deck still compiles exactly once.

src/main.ts

@@ -14,15 +14,17 @@ interface Slide {
     code: string
 }
 
-// The starter slide opens with a <style> tag. Less written inside any <style>
-// tag in the deck becomes the project stylesheet, and `:thisSlide` compiles to
-// each slide's own selector, so deck styles stay scoped without inventing any
-// extra CSS syntax.
+// The starter slide opens with a <style> tag. Less written inside a slide's
+// <style> tag is scoped to that slide, the global slide's styles are shared
+// by every slide, and `:thisSlide` is the slide's own section — no extra CSS
+// syntax beyond that.
 const STARTER_STYLE = `// Project styles. This is ordinary Less: variables, nesting,
 // mixins and imports all work as usual.
 //
-// :thisSlide compiles to the selector of the slide this stylesheet is being
-// compiled for, so it means "this slide", never "the visible slide".
+// A slide's own style tag is scoped to that slide: every rule in it applies
+// to that slide alone. The global slide's styles are shared by all slides.
+// :thisSlide is the slide's own section, never "the visible slide" — handy
+// for state rules: :thisSlide[data-ps-state='current'] { ... }.
 
 @bg: #101010;
 @fg: #f0f0f0;

src/slides/compile.ts

@@ -209,12 +209,13 @@ function isGlobalTemplate(global: string): boolean {
  * Compile a project into runtime-ready data.
  *
  * Every `<style>` tag in the deck contributes to one project stylesheet (the
- * global slide first, then each slide in order), and `:thisSlide` is rewritten
- * per style block *before* the blocks are joined: the global block to any
- * slide, each slide's block to that slide's own section. The joined source
- * then compiles exactly once — so Less keeps behaving like Less, the output
- * never depends on which slide is visible, and one slide's scoped rules can
- * never bind to its neighbours.
+ * global slide first, then each slide in order). The global block is the
+ * shared layer and compiles deck-wide; each slide's block is *wrapped* in its
+ * section selector before the blocks are joined, so a slide's stylesheet
+ * styles that slide and nothing else — token or not — with `:thisSlide`
+ * resolving to the section itself. The joined source compiles exactly once:
+ * Less stays Less, the output never depends on which slide is visible, and
+ * one slide's rules can never reach its neighbours.
  */
 export async function compilePresentation(input: CompileInput): Promise<CompiledPresentation> {
     const global = extractStyles(input.global ?? '')
@@ -227,15 +228,13 @@ export async function compilePresentation(input: CompileInput): Promise<Compiled
         html: globalIsTemplate ? slide.html : buildSlideHtml(slide.html, global.html)
     }))
 
-    // Bind `:thisSlide` while each block still belongs to exactly one slide:
-    // the global block means "whoever is showing", a slide block means that
-    // slide. Rebinding after the join (the old per-slide passes) rebound every
-    // block on every pass, so each slide's scoped rules matched all slides.
+    // Scope each block while it still belongs to exactly one slide: the
+    // global block means "everywhere", a slide block means that slide. Only
+    // rewriting the `:thisSlide` token (the previous fix) still left every
+    // untokened rule deck-wide — the whole block now carries the selector.
     const sources = [
         bindThisSlide(global.style, ANY_SLIDE_SELECTOR),
-        ...extracted.map((slide, index) =>
-            bindThisSlide(slide.style, slideSelector(slideId(index)))
-        )
+        ...extracted.map((slide, index) => scopeBlock(slide.style, slideSelector(slideId(index))))
     ]
     const source = sources.join('\n')
     const css = source.trim() ? await renderLess(source) : ''
@@ -246,3 +245,14 @@ export async function compilePresentation(input: CompileInput): Promise<Compiled
 function bindThisSlide(source: string, selector: string): string {
     return source.split(THIS_SLIDE).join(selector)
 }
+
+/**
+ * Scope a slide's whole style block to its section: wrap the block in the
+ * slide's selector and let `:thisSlide` become `&` — the section itself.
+ * Less hoists `@keyframes`/`@font-face` out of the wrapper on its own and
+ * keeps `@media` nested inside it, so only rules end up scoped.
+ */
+function scopeBlock(source: string, selector: string): string {
+    if (!source.trim()) return ''
+    return `${selector} {\n${source.split(THIS_SLIDE).join('&')}\n}`
+}