Can effects like Darkness, Light, etc. be strobed in a single turn?

Question

I've seen multiple questions (including one that I have written) that attempt some degree of maneuvering of an object or effect that would allow them to "strobe" the effects of spells like Light or Darkness, usually in some similar scenario:

Player: "For my Free Object Interaction, I'm going to cover this box/put away this marble/[do a thing which covers a Darkness effect]"
Player: Now that I can see, I'm going to use my Action to cast Fire Bolt at the enemy
Player: Now, because I multiclassed into three levels of Thief Rogue that can Use an Object as a Bonus Action, I'm going to use said Bonus Action to uncover this box/pull the marble back out/[do a thing which uncovers a Darkness effect], which will cause anyone who attacks me to have Disadvantage"

Commonly, posts like this receive some degree of scrutiny on the grounds that it's not possible to "strobe" effects because combat occurs simultaneously with other turns, and as a result the effect of this sequence of events, even if legal (in terms of their literal ability to perform the actions required), is to negate the benefits the play is intended to provide (Advantage on attacks/Disadvantage on enemy attacks, or cover for Hideing, or whatever).

But it's not clear to me, based on those posts, what the RAW (or even RAI) basis for this ruling is, other than DMs being worried about the exploitability of these kinds of plans. It seems like in many instances, the concept of Combat being Simultaneous is being interpreted as Combat being Instantaneous (or having an effect as the same).

(I don't want this post to be about whether or not these kinds of actions are, in fact, exploitable, or are as severe/benign as exploits as they seem to be treated, so please refrain from commenting on that specific debate; you can take it up with me in chat if that's what you care about)

So is there a clear set of rules (or interactions of rules) that forbid (or permit!) these kinds of tactics, or are these rulings mostly just a means of convenience for DMs trying to keep their players from breaking the power level of their games?

Answer 1

As far as I know, there are no rules that prevent this from being done. Also, I don't think the ruling is in spirits of the game. Reasoning below.

A typical combat encounter is a clash between two sides, a flurry of weapon swings, feints, parries, footwork, and spellcasting. The game organizes the chaos of combat into a cycle of rounds and turns. A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn.

So, while for imagination purposes and "flavor", you can think of the combat as simultaneous and chaotic, for gaming/mechanical purposes, the combat is indeed sequential.

But even if you want to make a ruling based on the flavor of a simultaneous, chaotic combat, it's still possible to have some kind of justification to this tactic, here is a suggestion that makes sense for me, for example, and how I would narrate it as a DM if someone questioned things like that: the nimbleness of the Thief allows him to quickly take out the box, shoot and then put the box back on before anyone can react to that - unless they were expecting it (i.e. they have readied an action). What I mean is: Exactly because the combat is chaotic, characters can't react in time to stuff their enemies are doing, unless they are ready for it.

On the other hand, unless the mentioned PC is a Warlock or has some thing that allows to see in Darkness, the disadvantage is negated by the fact that he can't see the attack either, so the enemy actually hits normally. This tactic is useful only when the enemy already has advantage through some other mean. I.e., even if the DM is worried about some kind of exploit here, there is virtually none.