Popcorn and cinemas and royal boxes.

Popcorn was an import from the Americas and a rather common snack across the New World. It did not start in the cinemas itself.

During the early decades of legitimate theatre (rather than when actors were considered reprobates and kept on the other side of the river) When the crown wanted to legitimise the theatres they had to be seen there. Which is the reason why the royal boz is placed on the side- where everyone can see that King George is in attendance and enjoying it.

However, plays go on for ages and to stop the king being hangry the theatre gave him a luxury snack of Popped Corn from the new world, coated with unrefined sugar (refined sugar being from the east and pretty rare) and so with people watching him eat the popcorn and clearly enjoying the luxury products of the soon to be ex-colonies. It became synonymous with enjoying the play in comfort. As with all luxury foods, it switched from being Box standard to the Groundlings snack of choice.