When Riptide Rex deals 6 damage to a 7 might Watcher at the battlefield, then the opponent moves a unit to the battlefield, is the Watcher healed before the showdown occurs?

Ruling: The Watcher is not healed before the showdown occurs. Units are healed at the end of combat (after combat damage is dealt) and at the end of the turn, but a combat doesn't begin until a unit moves to an occupied enemy battlefield, there is nothing on the chain, and no showdown is in progress.

Sequence:

• Riptide Rex deals 6 damage to the 7 might Watcher
• Opponent moves a unit to the battlefield
• Combat begins (no healing has occurred yet)
• The Watcher enters combat with 6 damage already on it