Each chat keeps its own effort level
A chat working in the background now summarises its history and runs its helpers at the effort it started with, not the one picked for the chat on screen.
Long chats summarise their earlier history so they can keep going. When several chats ran at once, a background chat's summary, and any helper it started, used the effort level set for whichever chat was open in the window.
Now the effort is fixed when a chat's turn starts and stays with that chat. Lowering effort for one chat no longer makes another one's summary thinner, and raising it no longer makes another one's helpers slower or costlier.