Fix computation of isLastSibling and isLastChild to account for muted or

otherwise hidden replies

src/state/queries/usePostThread/traversal.ts

@@ -193,7 +193,9 @@ export function sortAndAnnotateThreadItems(
                 storeTraversalMetadata(metadatas, childMetadata)
                 if (childParentMetadata) {
                   /*
-                   * Set this value before incrementing the parent's repliesIndexCounter
+                   * Set this value before incrementing the parent's
+                   * `repliesIndexCounter`, since `repliesIndexCounter` is
+                   * 1-indexed and `replyIndex` is 0-indexed.
                    */
                   childMetadata!.replyIndex =
                     childParentMetadata.repliesIndexCounter
@@ -273,6 +275,14 @@ export function sortAndAnnotateThreadItems(
               metadata.replyIndex ===
               metadata.parentMetadata.repliesIndexCounter - 1
 
+            /**
+             * Item is also the last "sibling" if its index matches the total
+             * number of replies we're actually able to render to the page.
+             */
+            const isLastSiblingDueToMissingReplies =
+              metadata.replyIndex ===
+              metadata.parentMetadata.repliesSeenCounter - 1
+
             /*
              * Item can also be the last "sibling" if we know we don't have a
              * next item, OR if that next item's depth is less than this item's
@@ -280,6 +290,7 @@ export function sortAndAnnotateThreadItems(
              * item).
              */
             const isImplicitlyLastSibling =
+              nextItem === undefined ||
               metadata.nextItemDepth === undefined ||
               metadata.nextItemDepth < metadata.depth
 
@@ -287,7 +298,9 @@ export function sortAndAnnotateThreadItems(
              * Ok now we can set the last sibling state.
              */
             metadata.isLastSibling =
-              isLastSiblingByCounts || isImplicitlyLastSibling
+              isLastSiblingByCounts ||
+              isImplicitlyLastSibling ||
+              isLastSiblingDueToMissingReplies
 
             /*
              * Item is the last "child" in a branch if there is no next item,
@@ -296,6 +309,7 @@ export function sortAndAnnotateThreadItems(
              * of this item)
              */
             metadata.isLastChild =
+              nextItem === undefined ||
               metadata.nextItemDepth === undefined ||
               metadata.nextItemDepth <= metadata.depth
 

src/state/queries/usePostThread/types.ts

@@ -200,12 +200,20 @@ export type TraversalMetadata = {
    *
    * After traversal, we can use this to calculate if we actually got all the
    * replies we expected, or if some were blocked, etc.
+   *
+   * Because this value is incremented startging from 0, it is 1-indexed. So to
+   * when comparing to the `replyIndex`, you'll need to subtract 1 from this
+   * value.
    */
   repliesSeenCounter: number
   /**
    * The total number of replies to this post hydrated in this response. Used
    * for populating the `replyIndex` of the post by referencing this value on
    * the parent.
+   *
+   * Because this value is incremented startging from 0, it is 1-indexed. So to
+   * when comparing to the `replyIndex`, you'll need to subtract 1 from this
+   * value.
    */
   repliesIndexCounter: number
   /**