Error management on required nested field

[olivier-robin31]

Here is a schema where I declare the childA as required (childA is nested under root2):

{
  "properties": {
    "root1": {
      "type": "string",
      "description": "Root 1"
    },
    "root2": {
      "type": "object",
      "description": "Root 2",
      "properties": {
        "childA": {
          "type": "string",
          "title": "Child A (required)"
        },
        "childB": {
          "type": "string",
          "title": "Child B (not required)"
        }
      },
      "required": [
        "childA"
      ]
    }
  }
}

When I "play" with it, I don't understand the error management.
2 consecutive use cases:

• both childA and childB are empty:
  => childA is in success ('has-success' and 'ng-valid' classes) although it must be filled => how can we indicate the problem on the field ?
  => root2 is in error ('has-error' class and error "message": "Missing required property: childA", "path": "#/childA")
  => the global form is valid => how can we disabled a submit button ?

• childA still empty, and childB is filled:
  => childA still in success
  => root2 still in error (with the same error)
  => root2 has now a second error "message": "Missing required property: childA", "path": "#/root2"
  => the global form is now invalid and has the error "message": "Missing required property: childA", "path": "#/childA"

It's very confusing:

• Why the required error is set on the parent of the field instead of the field itself like for other errors (minLength etc.) ?
• Why on the first level (not nested fields) there is not the problem ?
• Why the childA error is "locked" at the parent level (and is not raised at the global form) ? And why it is "unlocked" if childB is filled ?
• What is the correct way to manage errors ? In our custom widget code, shall we look at control.errors ? or errorMessages ? or formProperty._errors ?