I'm finally converting the question http://forum.zkoss.org/question/94358/onafterrender-not-called-after-grid-scrolling/ to a bug.
Grid has an onAfterRender event, which "Notifies one that the model's data has been rendered.", but this will be called only once, after the first elements were rendered. If the grid is longer than 50 elements, the 51..N elements won't be rendered when the event listeners will be attached to the elements.
It would be nice to maintain this lazy loading behavior (without having to render and send to the browser a big grid) and still being able to receive events for the new loaded elements.