fix symbol set hint when fill is a constant currentColor

[Fil]

when we specify fill: "currentColor", this.fill is undefined, but we still want to pass "currentColor" as a hint to the symbol scale

closes #1462

[mbostock]

Shouldn’t our handling of fill and stroke be the same?

[Fil]

My understanding is that fill defaults to "none", and "currentColor" for fill is transformed into null in style.js because that's the SVG default for path fill.

plot/src/style.js

Line 109 in 1ff36a3

mark.fill = impliedString(cfill, "currentColor");

but for the hint, we want to actually send an explicit fill: "currentColor" as the hint that this symbol has a fill.

The logic is different for stroke, which defaults to currentColor which is not transformed into null.

[mbostock]

The logic is different for stroke, which defaults to currentColor which is not transformed into null.

Right. But I think it’s still asymmetric. In essence, we’re trying to materialize the global default fill or stroke into the hint: if there isn’t a fill channel, nor this.fill, then we’re using the implied default fill which is currentColor. And for stroke, the implied default is none. So the logic should instead be:

      symbolChannel.hint = {
        fill: fillChannel
          ? fillChannel.value === symbolChannel.value
            ? "color"
            : "currentColor"
          : this.fill ?? "currentColor",
        stroke: strokeChannel
          ? strokeChannel.value === symbolChannel.value
            ? "color"
            : "currentColor"
          : this.stroke ?? "none"
      };