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?