VL 3.2 breaks multi mouseover selection handling?

[jheer]

Consider this example multi-selection spec:

• Prior to 3.2, shift-mouseover would color multiple points
• Post 3.2, shift-mouseover only ever highlights a single point

Looking at the generated Vega, the issue appears to stem from here:

{
  "name": "sel32_toggle",
  "value": false,
  "on": [
    {
      "events": [{"source": "scope", "type": "mouseover"}],
      "update": "event.shiftKey"
    },
    {"events": [{"source": "scope", "type": "mouseout"}], "update": "false"}
  ]
},
{
  "name": "sel32_modify",
  "update": "modify(\"sel32_store\", sel32_toggle ? null : sel32_tuple, sel32_toggle ? null : true, sel32_toggle ? sel32_tuple : null)"
}

Notice that, upon mouseout events, the sel32_toggle goes to false no matter what, even if shiftKey is true. If I replace false on mouseout with event.shiftKey the example appears to behave more reasonably, but I'm not 100% sure this is the right solution, as I'm assuming this bug was introduced while trying to resolve some other issue?

In any case this is breaking the interaction notebook I wrote that is going to students later this week. I may simply have to rewrite the notebook to avoid the issue, but in any case I'm flagging this as a high priority bug so that it gets attention and we can decide what needs to be done.

[jheer]

This issue also appears to have broken the documentation example here:
https://vega.github.io/vega-lite/docs/selection.html#conditional-encodings

[jheer]

Looking over the 3.2 release notes and files modified, I'm guessing the new selection clear functionality is the most likely culprit: #4720

Flagging @allenjlee!

[jheer]

Yup, found the root cause here:
https://github.com/vega/vega-lite/pull/4720/files#diff-76015606d4b117c2e2082fc707e94a07R47

It appears I can add clear: false to suppress the error, though I'd prefer not to have to do this. I'm not sure why we are using mouseout to automatically clear mouseover interactions by default.

[allenjlee]

A temporary solution right now is to set clear: false in the Vega-Lite specification under sel32 in your specification example. This will return to the appropriate behavior.

It seems like the problem is because we append the following to on in _toggle.

{"events": [{"source": "scope", "type": "mouseout"}], "update": "false"}

However, this is needed for other examples, specifically: https://vega.github.io/vega-lite/examples/interactive_query_widgets.html. I'm not sure what the best way to resolve this bug is, one way is to flag it so that it doesn't automatically add mouseout upon seeing mouseover for _toggle. What do you think @arvind?