score:1

Accepted answer

See my snippet - that‘s what I understood. You need to use JavaScript event and classes since CSS can‘t change state.

var svg = d3.select("svg");

var path = d3.geoPath();

d3.json("https://d3js.org/us-10m.v1.json", function(error, us) {
  if (error) throw error;

  svg.append("g")
    .attr("class", "counties")
    .selectAll("path")
    .data(topojson.feature(us, us.objects.counties).features)
    .enter()
    .append("path")
    .attr("d", path);

  // the following block is new, adding JS events
  let hoverEnabled = false;
  svg.on('mousedown', x => hoverEnabled = true)
    .on('mouseup', x => hoverEnabled = false)
  svg.selectAll('.counties path').on('mouseover', function() {
    if (hoverEnabled) {
      this.classList.add('hovered');
    }
  });
  
  svg.append("path")
     .attr("class", "county-borders")
    .attr("d", path(topojson.mesh(us, us.objects.counties, function(a, b) { return a !== b; })));
});
/* paths with class "hovered" need to be selected here, too */
.counties .hovered, .counties :hover {
  fill: red;
}

.county-borders {
  fill: none;
  stroke: #fff;
  stroke-width: 0.5px;
  stroke-linejoin: round;
  stroke-linecap: round;
  pointer-events: none;
}
<!DOCTYPE html>
<svg width="960" height="600"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v2.min.js"></script>


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