Can Ezreal use his Action-speed dash ability to move back to base after his attack trigger goes on the stack but before it resolves, and still have the attack trigger deal damage?

Ruling: You cannot activate Ezreal's Action-speed ability while his attack trigger is on the initial chain because Action abilities cannot be activated during a closed state (when a chain exists). Additionally, even if you could activate it, Ezreal's ability says "here" so the target would become illegal when Ezreal moves, causing the damage to fail on resolution.

Sequence:

• Ezreal moves to opponent's battlefield, triggering Showdown
• Ezreal's attack trigger goes on the initial chain
• Initial chain must resolve first (closed state - no Action abilities can be activated)
• After the attack trigger resolves and the chain closes, the attacker gets first focus
• Now you can activate Ezreal's dash ability to move him back to base
• Opponent can only respond with Reaction-speed effects at this point

Nuances:

• The "here" referential in Ezreal's ability checks on resolution, making targets illegal if Ezreal has moved
• Focus only passes when the chain closes (different from priority)
• This hit-and-run tactic of attacking then dashing back is the intended design for Ezreal