If Promising Future would start a showdown, does the showdown start before or after the rest of the card resolves?

Ruling: You cannot start a showdown while a chain exists. The showdown will stage and start at the first open state, which means after Promising Future fully resolves.

Sequence:

• Promising Future resolves completely
• The showdown stages
• The showdown starts at the first open state

Nuances:

• Players looking at their 5 cards cannot play those cards during the showdown staging, as the showdown doesn't actually start until the card fully resolves