scroll issues

[bakesteve]

Horizontal scroll

Need to check if affects main grid, or if focus sorts that. When keyboard navigating left in a regular canvas with horizontal scroll we don't fully move the header cell into view. We would ensure that by checking column.left + width vs node.scrollLeft

Vertical scroll

Happens automatically howdies to browser key events (up / down) but:

• doest seem to he testable
• not very explicit

A better option may be to move the state for what rows are visible up to the grid. Keep the rendered start/end at the viewport (its an optimisation)
Then we can update this state explicitly in navigateTo. Wed need to also move onScroll Upton the grid