Type Alias Hold 

pub type Hold = fn() -> bool;

Whether a host that presents frames of its own has every frame it owes on screen.

A renderer that paints outside the toolkit’s frame — an offscreen engine presented through a canvas, its frames arriving after an asynchronous read-back and decode — is invisible to the toolkit’s checkpoint: the checkpoint sees a committed frame, not that the host’s latest frame was in it. A capture taken then shows the canvas as it was, or empty. Such a host registers one hold (register_hold); the dayscript capture steps wait, bounded by the step’s timeout, while any hold answers false, before they request the checkpoint at all. Called on the UI thread, so the hold may read UI-thread state.