Implement additional keyboard navigation's path to cover this use case:
"When I use TAB the next cell is highlighted but it's not in edit mode, so I have to press enter to enter in edit mode. If I press TAB twice, I lose the cell focus (my browser bring the focus on the address bar!) I expect the same behavior of google sheet." (dmarrazz@redhat.com)

 ADDITONAL 

• Make popup editor (left click) fully operable with keyboard
• Make options menu (right click) fully operable with keyboard
• Always show focus indicator
• Correct the tab order
• Tab vs. Enter vs. Arrows to move around within grid