virtual-scroll: re-render when the viewport or content size changes

Add support for automatically re-rendering the list when the viewport or content size changes such that not enough content is rendered. We should also expose a public method to allow users to trigger this manually

Start by locating the virtual-scroll implementation and its existing rendering behavior. Trace how viewport and content sizes are observed, then review any existing virtual-scroll tests or public API entry points. Done means the list re-renders when either size changes leave insufficient content rendered, and users can trigger the same behavior through a public method.

Written by the indexing model from the issue text.