Runtime Event Policy

How an event becomes a message: the dispatch precedence the runtime applies.

Precedence

1. Global shortcuts — host/machine-declared global key handling
2. Declarative Tab traversal — Tab / Shift-Tab move focus across the scene-derived order, honoring scope policies (Wrap / Trap / Local / Passthrough)
3. Machine key_msg — the focused machine's raw-key handler gets a shot
4. Declarative fallback — activation (Enter on an on_activate node), focus events, ignore

The policy's design rule: common behavior is declarative; machines opt into raw keys deliberately. Tab is not delivered to machines as a raw key unless no declarative traversal applies — this is what makes modal trapping a scene declaration instead of per-modal key filtering.

RuntimeEvent

#![allow(unused)]
fn main() {
pub enum RuntimeEvent {
    Key(KeyEvent),      // key + ctrl/alt/shift
    Resize(ResizeEvent),
    Tick,
    Activate(NodeId),
    Focus(NodeId),
    Blur(NodeId),
    Sync(String),
}
}

dispatch(event) routes through the policy and, when routing yields a machine message, applies it through update exactly like send(msg). Focus-only outcomes update FocusState without touching Model.

Effects ride the same path

Effect::Emit re-enters update synchronously; Effect::RequestFocus re-enters dispatch. One send can therefore cascade through a whole message chain before returning — effects never queue for later and never reach the host.

For hosts

Translate your input layer into RuntimeEvent and nothing else. The demo's handle_runtime_key (raw host) and the notcurses host both do exactly this — see Embedding Knopper.