If a card has an attack trigger, can I move the unit into the showdown, then use a reaction spell to move it out while still keeping the attack trigger?

Yes, the trigger still resolves — but parts of it may do nothing depending on what it says.

Once a "when I attack" trigger is on the chain, it's an independent game object. Moving the unit away doesn't remove the trigger. However, whether the effect actually works depends on what the trigger says.

Rule 359.3.f.2 gives your exact scenario with an example: Yasuo, Remorseful attacks, his trigger goes on the chain, and an opponent moves him back to base with Fight or Flight in reaction. The ruling: "When the attack trigger resolves, 'here' is no longer the battlefield where combat is ongoing and the attack trigger mistargets." The trigger is on the chain and resolving, but the instruction referencing "here" can't find a valid location, so that instruction does nothing.

Two categories:

• No "here" / location reference — e.g., "When I attack, draw a card" or "When I attack, give other friendly units +1 Might this turn." These resolve fully regardless of where the unit went. The trigger is already on the chain and the instructions don't depend on the unit's presence.

• Uses "here" or a location-dependent phrase — e.g., "When I attack, deal 2 to an enemy unit here." The trigger resolves, but the "here" reference fails because the unit is no longer at that battlefield. That instruction is ignored (Rule 359.3.f.2—Yasuo example). Other independent instructions on the same trigger would still work.

One more nuance: Moving the unit to base (board zone) means effects that check "my Might" still work (the unit exists and has Might). Moving it to a non-board zone (hand, trash) makes characteristics return null — so "my Might" would also fail (Rule 359.3.e.12).