With fog-of-war, lighting, and scrying implemented, determining line of sight becomes just a check:

• Is there a static obstacle (box2d)?
• Are they visible (not hidden or in fog of war)?

Spells and autoattacks silently failing isn’t a great UX. Mages should move into position after the AttackCommand or CastCommand has been given.

Since I already have a MoveCommand, I wrote a moveIntoRange function for my SpellCastSystem if the target was out of range or move within line of sight.

if (isTargetOutbounds(wantsToCast)) {
    moveIntoRange(wantsToCast, castCommand);
}
// moveIntoRange just sends a MoveCommand to the target.
// This could cause issues if pathfinding isn't possible to that location
void moveIntoRange(Entity wantsToCast, CastCommand castCommand) {
    Position target = Comps.position.get(castCommand.target);
    wantsToCast.add(
        createComponent(MoveCommand.class).setup(target.x, target.y)
    );
}

However, for dynamic targets (like other entities), the MoveCommand wouldn’t update position until they arrived at the wrong location.

Spamming MoveCommands seemed like a bad idea given how expensive pathfinding could be, so I added a UpdatePathWithTolerance function that reissued a new MoveCommand when they drifted out of a static tolerance of ~0.5m.

As a bonus, I created an AutoTargetSystem that issues AttackCommand when an enemy enters line of sight. So now boarding is the survival horror it was always meant to be.