Casting a spell with a long casting time requires the caster to use their action each turn casting the spell:
you must spend your action each turn casting the spell
However, what would happen if while casting a long casting time spell (out of combat), a caster is suddenly attacked and is surprised?
Being surprised, the caster won't be able to use his action during his turn to continue casting the spell so is the spell forcefully lost? If it is lost, is the caster still considered to be concentrating on it up until his turn comes up?
This matters in particular for Pock, a level 12 War Magic Wizard who is ritually casting a tiny hut spell when his party gets ambushed. He would like to maintain the spell being cast as long as possible (or at least until he can actually take an action) to benefit from Durable Magic.