feat: rewrite to use app config and rework docs (#143)

Co-authored-by: Daniel Roe <daniel@roe.dev>
Co-authored-by: Sébastien Chopin <seb@nuxt.com>

docs/app/router.options.ts

@@ -0,0 +1,48 @@
+import type { RouterConfig } from '@nuxt/schema'
+
+function findHashPosition (hash): { el: any, behavior: ScrollBehavior, top: number } {
+  const el = document.querySelector(hash)
+  // vue-router does not incorporate scroll-margin-top on its own.
+  if (el) {
+    const top = parseFloat(getComputedStyle(el).scrollMarginTop)
+
+    return {
+      el: hash,
+      behavior: 'smooth',
+      top
+    }
+  }
+}
+
+// https://router.vuejs.org/api/#routeroptions
+export default <RouterConfig>{
+  scrollBehavior (to, from, savedPosition) {
+    const nuxtApp = useNuxtApp()
+
+    // If history back
+    if (savedPosition) {
+      // Handle Suspense resolution
+      return new Promise((resolve) => {
+        nuxtApp.hooks.hookOnce('page:finish', () => {
+          setTimeout(() => resolve(savedPosition), 50)
+        })
+      })
+    }
+
+    // Scroll to heading on click
+    if (to.hash) {
+      return new Promise((resolve) => {
+        if (to.path === from.path) {
+          setTimeout(() => resolve(findHashPosition(to.hash)), 50)
+        } else {
+          nuxtApp.hooks.hookOnce('page:finish', () => {
+            setTimeout(() => resolve(findHashPosition(to.hash)), 50)
+          })
+        }
+      })
+    }
+
+    // Scroll to top of window
+    return { top: 0 }
+  }
+}