When does a hidden opponent that is attacking become visible?

Question

The PHB p.180 specifies that during a round of combat, there are all manner of actions occurring during combat:

"In combat characters and monsters are in constant motion, often using movement and position to gain the upper hand."

It has been said that each round represents a theoretical six seconds in time during which a character finds an opportunity to take actions. The initiative roll is an abstraction representing the opportunity in the round that a participant gets into a position to execute an action (i.e. and take their "turn.") The idea that movement to open up opportunities is being taken throughout the round is also supported by the optional initiative rules in the DMG which allow for rolling initiative every round.

If movement is occurs throughout a round, this raises the question. When hidden opponents attack - when do they become visible? Do they become visible at the top of the round or only on their turn?

Using the top of the round would suggest that hidden opponents start moving and may not get an opportunity until later in the round before their action gets executed. Using only their turn would suggest they are able to stay hidden until they spring to make their attack.

Answer 1

The invisible or hidden condition ends when the attack hits or misses.

From PHB p. 194 - 195 regarding Unseen Attackers and Targets

If you are hidden -- both unseen and unheard -- when you make an attack, you give away your location when the attack hits or misses.

That means that your second interpretation fits better.

Using only their turn would suggest they are able to stay hidden until they spring to make their attack.

That takes the answer to your question away from an abstraction, and ties it to initiative/turn order. (D&D 5e is a turn based game, in its implementation of combat). It also preserves the advantage that accrues to such an attack.

the creature's attack rolls have advantage (p. 291)

Comparing the text of the invisible condition (p. 291), and the invisibility spell(p. 254), is consistent with the cite on unseen attackers and targets.

The spell ends for a target that attacks or casts a spell.

Subordinate question: will movement give the invisible/hidden PC away?

Maybe. Unless the PC moves, or declares a move, that question doesn't arise. (Unless you want to go out of your way to complicate things. Each table has their own preferences on that).

The chance of discovery depends on how soon in the initiative order that PC gets their turn, and what any other hostile creature does or doesn't accomplish via a passive or active perception check. It will be dependent on the situation.

For example, a creature like an Ancient Silver dragon, with a very high passive perception score, may detect the PC such that now that PC is no longer hidden from the silver dragon. That can render the entire effort moot, but regardless of whether or not the creature stayed hidden, the attack if the PC is invisible is (a) made with advantage (per the condition rules, p. 291) and (b) ends the invisible condition for the PC.