Can you play actions when a 'when attacking' chain is triggered, and does damage clean up when a showdown occurs due to someone taking an uncontested battlefield?

Ruling: While any chain is in progress (including chains with attacking/defending triggers), you can only play reactions, not actions. Units do not heal when someone moves to an empty battlefield; damage only clears at end of turn or if combat occurs.

Sequence:

• When a chain is triggered (such as "when attacking"), only reactions can be played
• Actions cannot be played during any active chain
• Damage persists on units unless combat occurs or end of turn is reached

Nuances:

• A showdown from taking an uncontested battlefield does not trigger damage cleanup
• Units with damage remain damaged until end of turn or actual combat, not from mere battlefield movement