fix(#9401): conditional opacity should respect default value (#9402)

## PR Description

Conditional opacity should respect default value


Before:

<img width="854" alt="image"
src="https://github.com/user-attachments/assets/07f590c3-a293-4e08-9d0d-d3b1dd2e15ef">

`config.point.opacity = 0.5` is ignored

After: 

<img width="794" alt="image"
src="https://github.com/user-attachments/assets/78e5b3ad-aa0f-40a2-8fe7-58f68a551fe7">

---------

Co-authored-by: GitHub Actions Bot <vega-actions-bot@users.noreply.github.com>

examples/compiled/point_conditional_opacity.vg.json

@@ -0,0 +1,113 @@
+{
+ "$schema": "https://vega.github.io/schema/vega/v5.json",
+ "background": "white",
+ "padding": 5,
+ "width": 200,
+ "height": 200,
+ "style": "cell",
+ "data": [
+ {
+ "name": "source_0",
+ "url": "data/cars.json",
+ "format": {"type": "json"},
+ "transform": [
+ {
+ "type": "filter",
+ "expr": "isValid(datum[\"Horsepower\"]) && isFinite(+datum[\"Horsepower\"]) && isValid(datum[\"Miles_per_Gallon\"]) && isFinite(+datum[\"Miles_per_Gallon\"])"
+ }
+ ]
+ }
+ ],
+ "marks": [
+ {
+ "name": "marks",
+ "type": "symbol",
+ "style": ["point"],
+ "from": {"data": "source_0"},
+ "encode": {
+ "update": {
+ "fill": {"value": "transparent"},
+ "stroke": {"value": "#4c78a8"},
+ "opacity": [
+ {"test": "datum.Horsepower > 100", "value": 0.25},
+ {"value": 0.5}
+ ],
+ "ariaRoleDescription": {"value": "point"},
+ "description": {
+ "signal": "\"Horsepower: \" + (format(datum[\"Horsepower\"], \"\")) + \"; Miles_per_Gallon: \" + (format(datum[\"Miles_per_Gallon\"], \"\"))"
+ },
+ "x": {"scale": "x", "field": "Horsepower"},
+ "y": {"scale": "y", "field": "Miles_per_Gallon"}
+ }
+ }
+ }
+ ],
+ "scales": [
+ {
+ "name": "x",
+ "type": "linear",
+ "domain": {"data": "source_0", "field": "Horsepower"},
+ "range": [0, {"signal": "width"}],
+ "nice": true,
+ "zero": true
+ },
+ {
+ "name": "y",
+ "type": "linear",
+ "domain": {"data": "source_0", "field": "Miles_per_Gallon"},
+ "range": [{"signal": "height"}, 0],
+ "nice": true,
+ "zero": true
+ }
+ ],
+ "axes": [
+ {
+ "scale": "x",
+ "orient": "bottom",
+ "gridScale": "y",
+ "grid": true,
+ "tickCount": {"signal": "ceil(width/40)"},
+ "domain": false,
+ "labels": false,
+ "aria": false,
+ "maxExtent": 0,
+ "minExtent": 0,
+ "ticks": false,
+ "zindex": 0
+ },
+ {
+ "scale": "y",
+ "orient": "left",
+ "gridScale": "x",
+ "grid": true,
+ "tickCount": {"signal": "ceil(height/40)"},
+ "domain": false,
+ "labels": false,
+ "aria": false,
+ "maxExtent": 0,
+ "minExtent": 0,
+ "ticks": false,
+ "zindex": 0
+ },
+ {
+ "scale": "x",
+ "orient": "bottom",
+ "grid": false,
+ "title": "Horsepower",
+ "labelFlush": true,
+ "labelOverlap": true,
+ "tickCount": {"signal": "ceil(width/40)"},
+ "zindex": 0
+ },
+ {
+ "scale": "y",
+ "orient": "left",
+ "grid": false,
+ "title": "Miles_per_Gallon",
+ "labelOverlap": true,
+ "tickCount": {"signal": "ceil(height/40)"},
+ "zindex": 0
+ }
+ ],
+ "config": {"style": {"point": {"opacity": 0.5}}}
+}

examples/specs/point_conditional_opacity.vl.json

@@ -0,0 +1,13 @@
+{
+ "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+ "data": {"url": "data/cars.json"},
+ "mark": "point",
+ "encoding": {
+ "x": {"field": "Horsepower", "type": "quantitative"},
+ "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
+ "opacity": {
+ "condition": [{"test": "datum.Horsepower > 100", "value": 0.25}]
+ }
+ },
+ "config": {"point": {"opacity": 0.5}}
+}

src/compile/mark/encode/nonposition.ts

@@ -1,6 +1,6 @@
 import type {SignalRef} from 'vega';
 import {NonPositionScaleChannel} from '../../../channel';
-import {Value} from '../../../channeldef';
+import {Value, isConditionalDef} from '../../../channeldef';
 import {VgEncodeChannel, VgEncodeEntry, VgValueRef} from '../../../vega.schema';
 import {getMarkPropOrConfig, signalOrValueRef} from '../../common';
 import {UnitModel} from '../../unit';
@@ -24,16 +24,22 @@ export function nonPosition(
  const {vgChannel} = opt;
  let {defaultRef, defaultValue} = opt;
 
+ const channelDef = encoding[channel];
+
  if (defaultRef === undefined) {
  // prettier-ignore
- defaultValue ??= getMarkPropOrConfig(channel, markDef, config, {vgChannel, ignoreVgConfig: true});
+ defaultValue ??= getMarkPropOrConfig(channel, markDef, config, {
+ vgChannel,
+ // If there is no conditonal def, we ignore vgConfig so the output spec is concise.
+ // However, if there is a conditional def, we must include vgConfig so the default is respected.
+ ignoreVgConfig: !isConditionalDef(channelDef)
+ });
 
  if (defaultValue !== undefined) {
  defaultRef = signalOrValueRef(defaultValue);
  }
  }
 
- const channelDef = encoding[channel];
  const commonProps = {
  markDef,
  config,