In a recent game we weren't entirely sure how certain spell areas would manifest when the origin of that spell is in a hallway, but it extends outside of the hallway.
As a Nymph-Blooded Sorcerer, I cast my Focus Spell, Blinding Beauty which has a 30-foot come. I was 10 feet into a hallway, aiming into a room with several enemies.
We weren't sure if the area would be entirely funnelled into only a straight line by the hallway, or if the cone would begin spreading from the point of exit of the hallway and create the equivalent of a 20-foot cone at the mouth of the hallway.
It didn't directly come up, but it also made us consider what would happen if someone cast something like Fireball in a hallway, with a portion of its area of effect extending out of the hallway. Would it just create a line of fire shooting straight out of the hallway or something like a half-circle with a radius of however much radius was left in the fireball?
Since the spell was a visual effect, we decided that anyone who could reasonably see could be effected, and that was fine for us, but I would like to know if there is an actual RAW answer.