Can a reaction be used against an attack from a hidden attacker? The rules say that the attacker gives up their hiding status when the attack hits or misses, not when the attack starts. [Top of page 195 of PHB]
A hidden rogue makes a ranged attack against a monk with deflect arrows. The monk can use his reaction to catch incoming arrows when they are hit. As far as I can tell, nothing in the rules prevents the monk from burning his reaction and trying to catch it, even if he doesn't see it coming. The only thing I can determine from page 96 of the PHB on sneak attack is that the rogue would get his sneak attack damage added to the damage the monk is trying to reduce.
A hidden caster of some type casts a spell at a sorcerer, warlock or wizard with counterspell. The spell description indicates that you need to see the spell coming in order to cast the counterspell. This seems to indicate that the mage in question could not in fact use counterspell in this situation, since they would not see the attacker until after the spell hit.
The way I read this is that SOME reactions can be used in a situation where stealth is in play, while others cannot, all based on the specifics of the situation. I was hoping to confirm this analysis.
Also, as noted below a surprise round may change the ruling for that particular round. I'm looking for a more general ruling - say several rounds into combat.