fix: added scroll preservation opt in to the compare page

app/pages/compare.vue

@@ -5,6 +5,7 @@ import FacetBarChart from '~/components/Compare/FacetBarChart.vue'
 
 definePageMeta({
   name: 'compare',
+  preserveScrollOnQuery: true,
 })
 
 const { locale } = useI18n()

app/router.options.ts

@@ -7,6 +7,17 @@ export default {
     if (savedPosition) {
       return savedPosition
     }
+
+    // Preserve the current viewport for query-only updates on pages that opt in,
+    // such as compare where controls sync state to the URL in-place.
+    if (
+      to.path === _from.path &&
+      to.hash === _from.hash &&
+      to.meta.preserveScrollOnQuery === true
+    ) {
+      return false
+    }
+
     // If navigating to a hash anchor, scroll to it
     if (to.hash) {
       const { scrollMargin } = to.meta

app/types/index.ts

@@ -10,5 +10,9 @@ declare module '#app' {
      * @default 70
      */
     scrollMargin?: number
+    /**
+     * preserve scroll position when only query params change on same path/hash
+     */
+    preserveScrollOnQuery?: boolean
   }
 }

test/unit/app/router.options.spec.ts

@@ -0,0 +1,73 @@
+import { describe, expect, it } from 'vitest'
+import routerOptions from '../../../app/router.options'
+
+type ScrollBehavior = NonNullable<typeof routerOptions.scrollBehavior>
+type RouteArg = Parameters<ScrollBehavior>[0]
+
+function createRoute(overrides: Partial<RouteArg> = {}) {
+  return {
+    path: '/',
+    hash: '',
+    query: {},
+    meta: {},
+    ...overrides,
+  } as RouteArg
+}
+
+describe('router scrollBehavior', () => {
+  it('restores saved position when available', () => {
+    const savedPosition = { left: 12, top: 345 }
+
+    expect(routerOptions.scrollBehavior(createRoute(), createRoute(), savedPosition)).toEqual(
+      savedPosition,
+    )
+  })
+
+  it('preserves scroll on query-only updates for pages that opt in', () => {
+    const to = createRoute({
+      path: '/compare',
+      query: { packages: 'vue,nuxt', facets: 'downloads,license' },
+      meta: { preserveScrollOnQuery: true },
+    })
+    const from = createRoute({
+      path: '/compare',
+      query: { packages: 'vue', facets: 'downloads' },
+      meta: { preserveScrollOnQuery: true },
+    })
+
+    expect(routerOptions.scrollBehavior(to, from, null)).toBe(false)
+  })
+
+  it('does not preserve scroll on query-only updates without opt-in', () => {
+    const to = createRoute({
+      path: '/compare',
+      query: { packages: 'vue,nuxt', facets: 'downloads,license' },
+    })
+    const from = createRoute({
+      path: '/compare',
+      query: { packages: 'vue', facets: 'downloads' },
+    })
+
+    expect(routerOptions.scrollBehavior(to, from, null)).toEqual({ left: 0, top: 0 })
+  })
+
+  it('scrolls to hash anchors', () => {
+    const to = createRoute({
+      hash: '#section-function',
+      meta: { scrollMargin: 96 },
+    })
+
+    expect(routerOptions.scrollBehavior(to, createRoute(), null)).toEqual({
+      el: '#section-function',
+      behavior: 'smooth',
+      top: 96,
+    })
+  })
+
+  it('scrolls to top for regular navigations', () => {
+    const to = createRoute({ path: '/compare', meta: { preserveScrollOnQuery: true } })
+    const from = createRoute({ path: '/search' })
+
+    expect(routerOptions.scrollBehavior(to, from, null)).toEqual({ left: 0, top: 0 })
+  })
+})