Viewport and accessibility

The viewport layer handles virtualization, visible row calculation, and DOM focus coordination.

1) Core concepts

• Viewport — visible area rectangle.
• Row projection — visible row range for render.
• Column projection — visible column set.

2) Invalidation and refresh

Core provides explicit sync points:

api.refreshRows("manual")
api.refreshColumns("manual")

Use them when:

• row heights change,
• columns are rebuilt,
• container size changes.

3) A11y contract

Recommended minimum in the adapter:

• a single active focus (roving tabindex),
• aria role for the grid container,
• aria‑col/row for cells,
• focus handling on scroll/selection.

4) Common mistakes

• Rendering extra rows without projection sync.
• Interrupted measurements during frequent resize.
• Focus loss on selection updates.