feat(theme): un-deprecate Theme.setup and compose it across extends (#5404)

docs/en/guide/custom-theme.md

@@ -38,7 +38,12 @@ interface Theme {
    */
   enhanceApp?: (ctx: EnhanceAppContext) => Awaitable<void>
   /**
-   * Extend another theme, calling its `enhanceApp` before ours
+   * Runs inside the root component's `setup()`
+   * @optional
+   */
+  setup?: () => void
+  /**
+   * Extend another theme, calling its `enhanceApp` and `setup` before ours
    * @optional
    */
   extends?: Theme
@@ -88,6 +93,26 @@ export default {
 
 Return `false` from `onBeforeRouteChange` or `onBeforePageLoad` to cancel navigation.
 
+The `setup` hook runs inside the root component's `setup()`, so Composition API calls (`onMounted`, `watch`, composables, ...) work there without wrapping the layout component:
+
+```ts [.vitepress/theme/index.ts]
+import { watch } from 'vue'
+import { useData } from 'vitepress'
+import DefaultTheme from 'vitepress/theme'
+
+export default {
+  extends: DefaultTheme,
+  setup() {
+    const { page } = useData()
+    watch(() => page.value.relativePath, (path) => {
+      console.log('now viewing', path)
+    })
+  }
+}
+```
+
+With `extends`, each theme's `setup` runs base-first, like `enhanceApp`. It also runs during SSR/SSG rendering, so keep browser-only work inside `onMounted`.
+
 The default export is the only contract for a custom theme, and only the `Layout` property is required. So technically, a VitePress theme can be as simple as a single Vue component.
 
 Inside your layout component, it works just like a normal Vite + Vue 3 application. Do note the theme also needs to be [SSR-compatible](./ssr-compat).

src/client/app/index.ts

@@ -26,8 +26,12 @@ function resolveThemeExtends(theme: typeof RawTheme): typeof RawTheme {
       ...base,
       ...theme,
       async enhanceApp(ctx) {
-        if (base.enhanceApp) await base.enhanceApp(ctx)
-        if (theme.enhanceApp) await theme.enhanceApp(ctx)
+        await base.enhanceApp?.(ctx)
+        await theme.enhanceApp?.(ctx)
+      },
+      setup() {
+        base.setup?.()
+        theme.setup?.()
       }
     }
   }

src/client/app/theme.ts

@@ -15,7 +15,8 @@ export interface Theme {
   extends?: Theme
 
   /**
-   * @deprecated can be replaced by wrapping layout component
+   * Runs inside the root component's `setup()` (during SSR too). With
+   * `extends`, setups run base-first, like `enhanceApp`.
    */
   setup?: () => void