Fix isPartOfLastBranchFromDepth assignment (#8856)

src/state/queries/usePostThread/traversal.ts

@@ -307,9 +307,16 @@ export function sortAndAnnotateThreadItems(
               metadata.isPartOfLastBranchFromDepth = metadata.depth
 
               /**
-               * If the parent is part of the last branch of the sub-tree, so is the child.
+               * If the parent is part of the last branch of the sub-tree, so
+               * is the child. However, if the child is also a last sibling,
+               * then we need to start tracking `isPartOfLastBranchFromDepth`
+               * from this point onwards, always updating it to the depth of
+               * the last sibling as we go down.
                */
-              if (metadata.parentMetadata.isPartOfLastBranchFromDepth) {
+              if (
+                !metadata.isLastSibling &&
+                metadata.parentMetadata.isPartOfLastBranchFromDepth
+              ) {
                 metadata.isPartOfLastBranchFromDepth =
                   metadata.parentMetadata.isPartOfLastBranchFromDepth
               }

src/state/queries/usePostThread/types.ts

@@ -151,8 +151,8 @@ export type TraversalMetadata = {
    */
   isLastChild: boolean
   /**
-   * Indicates if the post is the left/lower-most branch of the reply tree.
+   * Indicates if the post is the left-most AND lower-most branch of the reply
-   * Value corresponds to the depth at which this branch started.
+   * tree. Value corresponds to the depth at which this branch started.
    */
   isPartOfLastBranchFromDepth?: number
   /**