Melee with AOE-impacted enemy

Question

I'm thinking specifically about the D&D 5th edition Moonbeam spell. Given that we don't play with a grid, and players are free to place the centre of the beam anywhere they like, there is a desire to place it such that an enemy is in the beam, while player characters they are in combat with are not in the beam.

There are two specific situations I want to consider and how they might play out.

Firstly, assuming the combatants aren't using ranged weapons, or weapons with unusual reach, does attacking the AOE-engulfed target with a melee weapon result in the attacker's hands potentially entering the AOE and thus incurring damage?

Secondly, given that combat is not really static, with each combatant making a single swing in their allotted 6 second turn, is it more realistic (for want of a better word) to assume that two combatants next to each other, engaged in dynamic melee combat are inseparable for constant-effect AOE spells? (I can imagine a situation where an instant-effect like fireball can be arranged not to hit your friends, so I'm concentrating here on constant-effect spells like Moonbeam).

So the question is, can constant-effect spells like Moonbeam be placed in such a way that two combatants, in melee combat with each other, can be on either side of the AOE, one affected and one not for the entire duration?

Answer 1

A decent amount of AOE effects take place when a creature enters or starts their turn in a square/space affected by the spell.

So, at what point does a creature enter a square? Typically, the mechanics of the game trigger when the creature steps completely into the square, because on the grid system there are no half-square steps, it's completely discrete movement. In this case, I would argue that attacking a creature from outside the AOE wouldn't hurt the attacker, ever.

However, when not playing on a grid, things can be a bit more uncertain, but they can also be cut a lot more, depending on how you play. I would, IMHO, still rule that a person didn't actually enter the AOE until a significant amount of their body is in the AOE (ie, at least 10% of their body or your arbitrary number of choice) or however you want to rule it. Following that rule, characters using weapons wouldn't get hit, and characters using their fists MIGHT get hit, depending on how far in they have to reach to attack.

Also make sure that you inform your pc's about this change before implementing it, just like all other house rules.