Can't tab off ComboBox button

[Bargs]

I can't reproduce this in the demo site, only in my filter bar PR, so I'm not sure what interaction is causing it. It also seems to be an issue specific to single selection mode.

How to reproduce:

1. Open the Add Filter editor.
2. Select field, operator
3. Click the value combobox. Select a value using the keyboard and hit enter.
4. Press tab. Notice it focuses the downward caret button to the right.
5. Keep hitting tab, focus doesn't change.
   (bonus steps)
6. Press ESC
7. Press tab, now the focus moves correctly.

Here's a gif of me doing steps 1 - 5:

[Bargs]

@chandlerprall

[thompsongl]

Appears to be a legit bug with the single selection. I was able to reproduce in the EUI docs site. The problem appears after making a selection via keyboard and attempting to tab away, and only for the single selection version.