Answer
On supporting web, iOS and Android builds, a label such as +3 more items tells you how many rows are hidden. The label is informational. Select Show Full Message beneath the message to reveal the rest of the list, accompanying text and hidden previews together. Hide Full Message returns to the four-item preview.
Visible checkboxes remain usable while the message is collapsed. Checking an item keeps the current expansion state and item order. Forwarded checklists follow the same preview rule.
Lists with four or fewer items show all rows. Private AI chats, Temporary Chats and search results also show every row. If you asked for the checklist, you see every row while you stay in the chat after asking. Existing message positioning and reduced-motion behavior still apply.