Vega tooltips do not work

Simple example that works in the online Vega editor but not in the "vega spec transition" example:

{
  "$schema": "https://vega.github.io/schema/vega/v3.json",
  "width": 200,
  "height": 200,
  "data": [
    {
      "name": "table",
      "values": [
        {"x": 12, "y": 23},
        {"x": 22, "y": 3}
      ]
    }
  ],
  "scales": [
    {
      "name": "xscale",
      "type": "linear",
      "domain": {"data": "table", "field": "x"},
      "range": "width"
    },
    {
      "name": "yscale",
      "type": "linear",
      "domain": {"data": "table", "field": "y"},
      "range": "height"
    }
  ],
  "marks": [
    {
      "type": "rect",
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "x": {"field": "x", "scale": "xscale"},
          "y": {"field": "y", "scale": "yscale"},
          "width": {"value": 30},
          "height": {"value": 30},
          "tooltip": {"signal": "datum.x"}
        },
        "update": {"fill": {"value": "#ccc"}},
        "hover": {"fill": {"value": "pink"}}
      }
    }
  ],
  "config": {}
}

How have you implemented tooltips in the SandDance explorer if they don't work in Vega? I was under the impression that it used Vega under the hood - is that not the case?

Also weirdly the hover fill is always black.

Start with the "vega spec transition" example and compare its behavior with the online Vega editor using the supplied specification. Investigate how the example handles the tooltip signal and hover fill, then verify that tooltips respond and the hover fill is not rendered black.