An interesting combination emerged in the gameplay: An invisible party member decided to defend another party member using his reaction without spells and no attacks (arrow-marksmen).
This led to two questions:
In the description for the invisibility Spell (PH p. 254), states:
The spell ends for a target that attacks or acts on a spell.
So this led to the question – Does this reaction end with invisibility?
In the description for the Backstop plate, it says
Whenever an attacker makes a ranged attack on a
Target within 5 feet of you, you can use your reaction to that
Target of the attack instead.
In fact, there is a long list of reactions without spells that can not be attacked (Bardean inspiration, misdirection, etc.) that can influence an attack.
Question 1: Does this reaction cause the invisibility spell to stop?
The consensus at the table was "No, it's neither a spell nor an attack."
But wait, there is more!
According to the rules of invisible Condition (PHB, p. 291):
Attack rolls against you have disadvantage.
Theoretically, the attack should now be penalized even though it was already done.
This made us all scratch our heads (and the normal attack was missing anyway, so we continued)
Question 2: Is rerouting an attack on an invisible target now leading to an offensive attack?