Preserve thread numbering in placeholders (#11616)

src/lib/api/feed-manip.test.ts

@@ -0,0 +1,54 @@
+import {type app} from '#/lexicons'
+import {createFeedViewPostsSlices} from './feed-manip'
+
+jest.mock('./feed/home', () => ({
+  FALLBACK_MARKER_POST: {post: {uri: 'at://did:plc:test/app.bsky.feed.post/1'}},
+}))
+
+const author = {
+  $type: 'app.bsky.actor.defs#profileViewBasic',
+  did: 'did:plc:alice',
+  handle: 'alice.test',
+} as app.bsky.actor.defs.ProfileViewBasic
+
+function post(id: string) {
+  return {
+    $type: 'app.bsky.feed.defs#postView',
+    uri: `at://did:plc:alice/app.bsky.feed.post/${id}`,
+    cid: id,
+    author,
+    record: {
+      $type: 'app.bsky.feed.post',
+      text: id,
+      createdAt: '2026-08-31T00:00:00.000Z',
+    },
+    indexedAt: '2026-08-31T00:00:00.000Z',
+  } as app.bsky.feed.defs.PostView
+}
+
+describe('createFeedViewPostsSlices', () => {
+  it('preserves selected numbering and infers hydrated parent and root numbering', () => {
+    const root = post('root')
+    const parent = post('parent')
+    const selected = post('selected')
+    const feedPost = {
+      post: selected,
+      reply: {root, parent},
+      opThreadPostIndex: 3,
+      opThreadPostCount: 4,
+    } as app.bsky.feed.defs.FeedViewPost & {
+      opThreadPostIndex: number
+      opThreadPostCount: number
+    }
+
+    const [slice] = createFeedViewPostsSlices([feedPost])
+
+    expect(
+      slice.items.map(item => [item.post.uri, item.postNumbering]),
+    ).toEqual([
+      [root.uri, {opThreadPostIndex: 1, opThreadPostCount: 4}],
+      [parent.uri, {opThreadPostIndex: 2, opThreadPostCount: 4}],
+      [selected.uri, {opThreadPostIndex: 3, opThreadPostCount: 4}],
+    ])
+  })
+})

src/lib/api/feed-manip.ts

@@ -9,7 +9,7 @@ export type FeedPostNumbering = Pick<
   'opThreadPostIndex' | 'opThreadPostCount'
 >
 
-type ValidFeedPostNumbering = Required<FeedPostNumbering>
+export type ValidFeedPostNumbering = Required<FeedPostNumbering>
 
 // AppView adds these fields to feed responses ahead of their feed lexicon.
 type FeedViewPost = app.bsky.feed.defs.FeedViewPost & FeedPostNumbering
@@ -62,7 +62,7 @@ export type FeedTunerFn = (
 type FeedSliceItem = {
   post: app.bsky.feed.defs.PostView
   record: app.bsky.feed.post.Main
-  postNumbering: FeedPostNumbering | undefined
+  postNumbering: ValidFeedPostNumbering | undefined
   parentAuthor: app.bsky.actor.defs.ProfileViewBasic | undefined
   isParentBlocked: boolean
   isParentNotFound: boolean
@@ -88,7 +88,7 @@ export class FeedViewPostsSlice {
 
   constructor(
     feedPost: FeedViewPost,
-    postNumberingByUri: Map<string, FeedPostNumbering>,
+    postNumberingByUri: Map<string, ValidFeedPostNumbering>,
   ) {
     const {post, reply, reason} = feedPost
     this.items = []
@@ -286,6 +286,22 @@ export class FeedViewPostsSlice {
   }
 }
 
+export function createFeedViewPostsSlices(
+  feed: FeedViewPost[],
+): FeedViewPostsSlice[] {
+  const postNumberingByUri = new Map<string, ValidFeedPostNumbering>()
+  for (const item of feed) {
+    const postNumbering = getPostNumbering(item)
+    if (postNumbering) {
+      postNumberingByUri.set(item.post.uri, postNumbering)
+    }
+  }
+
+  return feed
+    .map(item => new FeedViewPostsSlice(item, postNumberingByUri))
+    .filter(slice => slice.items.length > 0 || slice.isFallbackMarker)
+}
+
 export class FeedTuner {
   seenKeys: Set<string> = new Set()
   seenUris: Set<string> = new Set()
@@ -299,17 +315,7 @@ export class FeedTuner {
       dryRun: false,
     },
   ): FeedViewPostsSlice[] {
-    const postNumberingByUri = new Map<string, FeedPostNumbering>()
-    for (const item of feed) {
-      const postNumbering = getPostNumbering(item)
-      if (postNumbering) {
-        postNumberingByUri.set(item.post.uri, postNumbering)
-      }
-    }
-
-    let slices: FeedViewPostsSlice[] = feed
-      .map(item => new FeedViewPostsSlice(item, postNumberingByUri))
-      .filter(s => s.items.length > 0 || s.isFallbackMarker)
+    let slices = createFeedViewPostsSlices(feed)
 
     // run the custom tuners
     for (const tunerFn of this.tunerFns) {