Describe the feature:
Current Kibana: 7.12.1 running in ESS

Current workflow:

1. Go to transforms
2. Select an (existing) transform
3. Change to the preview tab
4. Now select the columns to be shown.
5. Switch to another tab within the transform
6. The column selection is not preserved.

Expected workflow:

Point 6, should have persisted the selected columns shown for the preview.

Possible further improvement:

Display impacted columns:

Parse the transforms JSON job and only display the impacted columns.
E.g. the following transform job gets the latest value of currency.

{
  "id": "latest-currency-value",
  "version": "7.12.1",
  "create_time": 1620815500659,
  "source": {
    "index": [
      "currencies"
    ],
    "query": {
      "match_all": {}
    }
  },
  "dest": {
    "index": "currency-latest-value"
  },
  "frequency": "15m",
  "sync": {
    "time": {
      "field": "@timestamp",
      "delay": "10m"
    }
  },
  "latest": {
    "unique_key": [
      "currency"
    ],
    "sort": "@timestamp"
  },
  "settings": {
    "max_page_search_size": 500
  }
}

Display not null/empty columns

If a column is empty automatically deselect it.