Preview and simulate
Preview renders one message as a chosen contact would receive it. Simulate goes further: it runs the cascade for that contact and lists the sequence they would get: pool, message, position, and why each was chosen or skipped.
They use the same engine as sending, so if Simulate shows it, that is what will send.
Edit a message from the preview
Edit message, beside the pool name, opens the same field editor as the message page. Save, and the preview shows the change at once.
A message can sit in more than one pool, and a pool in more than one campaign. When the message you are editing is used anywhere else, the preview says where before the editor opens, and asks again before it saves, because the edit changes it everywhere it is used.
Versions (block variants)
A step whose row has variants shows N versions in the list and a Version choice above the message. With no contact, it shows Default until you pick another; the choice stays as you move through the steps that have it. With a contact, it shows the version that contact gets and why; you can still pick another to compare, marked as not theirs.
No waiting between messages
When you open a message, the rest of its page and the whole next page load behind it, so stepping through with the arrow keys does not stop to load each one.