[APP-1398] Fix post shadow/like shadow state when replying (#8866)

* Add optimisticReplyCount to post shadow

* Add special util

* Fix subtle bug in query cache

src/state/queries/usePostThread/queryCache.ts

@@ -9,6 +9,10 @@ import {
 } from '@atproto/api'
 import {type QueryClient} from '@tanstack/react-query'
 
+import {
+  dangerousGetPostShadow,
+  updatePostShadow,
+} from '#/state/cache/post-shadow'
 import {findAllPostsInQueryData as findAllPostsInExploreFeedPreviewsQueryData} from '#/state/queries/explore-feed-previews'
 import {findAllPostsInQueryData as findAllPostsInNotifsQueryData} from '#/state/queries/notifications/feed'
 import {findAllPostsInQueryData as findAllPostsInFeedQueryData} from '#/state/queries/post-feed'
@@ -85,10 +89,27 @@ export function createCacheMutator({
           /*
            * Update parent data
            */
-          parent.value.post = {
-            ...parent.value.post,
-            replyCount: (parent.value.post.replyCount || 0) + 1,
-          }
+          const shadow = dangerousGetPostShadow(parent.value.post)
+          const prevOptimisticCount = shadow?.optimisticReplyCount
+          const prevReplyCount = parent.value.post.replyCount
+          // prefer optimistic count, if we already have some
+          const currentReplyCount =
+            (prevOptimisticCount ?? prevReplyCount ?? 0) + 1
+
+          /*
+           * We must update the value in the query cache in order for thread
+           * traversal to properly compute required metadata.
+           */
+          parent.value.post.replyCount = currentReplyCount
+
+          /**
+           * Additionally, we need to update the post shadow to keep track of
+           * these new values, since mutating the post object above does not
+           * cause a re-render.
+           */
+          updatePostShadow(queryClient, parent.value.post.uri, {
+            optimisticReplyCount: currentReplyCount,
+          })
 
           const opDid = getRootPostAtUri(parent.value.post)?.host
           const nextPreexistingItem = thread.at(i + 1)