- and use this to determine whether or not the entire grid should have a tabindex/be focusable, rather than a simple 'hasFocus' check which doesn't account for virtualization

- Add new mockFocusContext for easier testing for various tests that need to set a focus context

- EuiDataGridCell
  - DRY out an `this.isFocusedCell` method
  - Call `setIsFocusedCellInView` on mount and on unmount (`setFocusedCell` handles setting true/false on new cell focus)
  - Write new unit tests for componentWillUnmount
  - Clean up unnecessary uncovered function fallback for this.unsubscribeCell - we're already checking for a falsy value before calling it

…ntent

- see http://localhost:8030/#/tabular-content/data-grid-focus

…irtualized refs

- see https://react-window.vercel.app/#/api/FixedSizeGrid for `onItemsRendered` API
- this struck me as the quickest/easiest way to determine which virtualized cells are in view
- I opted to save this as a ref instead of as state as I didn't see the need to cause a rerender

- `focusFirstVisibleInteractiveCell` was split out to its own fn as it will shortly be used elsewhere to fix another focus bug in the grid

…umn via header

…er actions

…lling content"

This reverts commit 7f031ac.

- gridItemsRendered will be falsey if rowCount is 0 and will cause an error on tab, so we should opt to simply leave focus on the grid body wrapper

…t in view