The focused element in command blocks is not well defined. To reproduce:

1. Place and open a command block
2. Press the O button to toggle its previous output field display. That button should now be highlighted.
3. Hit the space bar and observe the field display be toggled
4. Hit tab and observe command suggestions update
5. Repeat steps 3 and 4 at will

Expected behavior would that only one of these UI elements is in focus at one time.