One Keypress, Fully Traced

Consider an insert-mode key that changes the current document.

1. The event loop wins the terminal branch

Application::event_loop_until_idle uses a biased tokio::select! over signals, terminal input, job callbacks, status messages, waited jobs, and editor events. This is multiplexing: one task waits for many sources without dedicating a thread to each one.

2. The compositor routes the event

handle_terminal_events constructs a short-lived Context borrowing &mut Editor and &mut Jobs, then calls Compositor::handle_event. Layers are visited front to back until one consumes the event. Deferred layer callbacks run only after traversal, avoiding a second mutable borrow of the layer stack during iteration.

3. A command describes an edit

Editing code produces a Transaction, not a sequence of incidental string mutations. A transaction contains a ChangeSet and an optional new selection. It can be composed and inverted, which makes undo and coordinate mapping part of the editing model.

4. The document commits the consequences together

Document::apply_impl applies the changes to the rope, increments the version, maps every view’s selection and anchor through the changes, composes savepoint inverses, updates syntax state, and emits change notifications. A command cannot update text yet forget that cursor positions live in the old coordinate space.

5. Redraw is a result, not a second editor

If handling requests a redraw, the application renders the compositor into a surface and asks the terminal to draw it. Other redraw requests are coalesced through a timer in Editor::wait_event.

The complete invariant is: input may arrive concurrently, but each committed edit and render observes exclusive access to the model.