Keyboard navigation/focus broken near header

[sebbarg]

There is a bug in keyboard navigation when standing on the first row and pressing up-arrow on the keyboard. You are no longer able to select the focused row with enter. Press down-arrow once and it works again.

Repro:

1. Go here: https://reactdatagrid.io/demo
2. Click the first row (focus rect is visible, row is selected)
3. Keyboard: arrow key down (focus rect moves to second row, first row is still selected)
4. Keyboard: enter (focus rect stays, selection moves to second row)
5. Keyboard: arrow key up (focus rect moves to first row, second row is still selected)
6. Keyboard: arrow key up (focus rect stays, second row is still selected)
7. Keyboard: enter (NOTHING HAPPENS - first row should have been selected here)

Something is off when pressing arrow-up in (6)

This is a problem when doing multi-selects from the keyboard.