name: VirtualScroll

purpose: Variable-height virtual scrolling utility used by List and DataGrid.

factory:
  signature: VirtualScroll(options)
  options:
    itemHeight: Optional fixed initial item height.

methods:
  reset: Initialize a new list and return its first render window.
  init: Measure initial data and invoke refresh callback.
  update: Measure one rendered item.
  calc: Return paddings, renderedList, and listStartIndex for current scroll position.

rules:
  - Treat this as an internal UI utility unless implementing a virtualized component.
  - Call update after each rendered row can be measured.

examples:
  basic: |
    const virtual = VirtualScroll({ itemHeight: 40 })
    virtual.reset(rows, container)
    virtual.init(rows, () => refresh())
    const view = virtual.calc(container, rows)

tests:
  - ../components/base/List.test.html
  - ../components/data/DataGrid.test.html