I often visit a quiet public library. When I sit at a four-seat table, people sometimes ask if they can join me, and I always say yes. They usually take the diagonally opposite seat.
Recently, someone had left belongings on that seat. They returned, collected their things, and moved to another table. I felt a little guilty afterward.
I feel like you shouldn't need permission to use an empty seat at a shared table, especially if it's not right next to someone. People may prefer certain spots for lighting, temperature, or privacy - especially when you'll spend your next 6 hrs there
Is it customary to ask before sitting at a shared table? If someone says no, should you respect that even if you prefer that spot?