Signal scale updates on initialization

I would like to have a signal that computes scaled coordinates once, when the scale is initialized with a valid domain and range, and then no longer reacts to any further updates to the scale.

In the following spec, if react is true, only a single warning is printed which suggests that the signal is correctly marked as dependent on the scale. However, setting react to false produces a single [undefined, undefined] which suggests that the signal is no longer being topologically evaluated after the scales are initialized? It’s unclear to me if this is a bug or by design and, if the latter, how I would accomplish my desired outcome.

{
  "$schema": "https://vega.github.io/schema/vega/v4.json",
  "width": 200,
  "height": 200,
  "data": [
    {
      "name": "source_0",
      "url": "data/cars.json"
    }
  ],
  "signals": [
    {
      "name": "box_x",
      "update": "warn('box_x', [scale(\"x\", 100),scale(\"x\", 150)])",
      "react": false
    }
  ],
  "scales": [
    {
      "name": "x",
      "type": "linear",
      "domain": {"data": "source_0", "field": "Horsepower"},
      "range": [0, {"signal": "width"}]
    }
  ]
}

[jheer]

There are internal use cases for bridging event streams with the dataflow graph, but no JSON-spec level use cases I know of other than initialization.

[jheer]

Addressed with the init signal property, released in Vega v4.4.0.