[Discover] Focus behavior in field list is confusing

[wylieconlon]

I was confused by the behavior of the focus state in Discover when adding/removing fields from the columns. This behavior might be intentional, but it confused me enough to note here.

As seen in this gif, I followed these steps:

1. Click "add" on a field in discover
2. The field I just clicked moves from under my mouse, but the focus state stays
3. Realize that the focus state didn't move with the field, and is instead focused on the next unselected field

The main confusion here is that the list updates immediately after clicking. It's worse when scrolled far down the list of fields, because there's no visual indicator of what happened.

To suggest a fix: don't remove fields from the "available fields" list when they are interacted with. This would instead shift the entire layout after I click "add", which would clearly indicate to me that my change had worked.

cc @kertal @andreadelrio

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)