Function for_a_client 

fn for_a_client(update: SessionUpdate, cap: usize) -> SessionUpdate

The update as it crosses to a client.

A seat’s transcript never rides an update to a client. The two variants that carry one are the session task’s seed for the transport’s own copy, and a client answers them with a READ - the subscribe snapshot’s windowed conversation and more above it (the client’s own REPLACES list says so, client/src/session/apply.ts) - rather than drawing what they carry. So the message list is emptied here, at the one place an update becomes something a client hears; what stays is the seat’s session facts.

The bound has to be the frame’s, not the window’s. A window is bounded by the ceilings a seat keeps it in (frames and bytes, the module beside the task), and those are a seat’s own business: what a live 2.5GiB seat’s resume did to a connection (#2150) was ride one Connected as a single 2.46GiB frame, refused at the 256MiB one the client’s socket carries (MAX_FRAME_BYTES, client/src-tauri/socket.rs). What a client reads instead is a page, and a page is bounded by turns on top of whatever the window holds (twenty a subscribe, twenty a more), so its size leans on that frame cap rather than sitting orders under it. A frame whose size does not depend on the conversation’s size is the only one that holds whatever the seat weighs.