drag.filter is awkward.

[mbostock]

In the case of using d3-drag to drag nodes with Canvas, you might want to use drag.filter to check whether a node was clicked on before allowing the drag behavior. However, there’s no way to access whatever is computed during filter on dragstart (aside from having the caller use a global). For example, you might hit-test your circles:

function filter() {
  var node, i, x, y, n = nodes.length;
  for (i = 0; i < n; ++i) {
    node = nodes[i];
    x = d3.event.x - node.x;
    y = d3.event.y - node.y;
    if (x * x + y * y < 25) return node;
  }
}

Unfortunately, you’d have to perform this same hit-test again to determine the current node on dragstart.

Maybe drag.filter should be replaced by drag.target, which sets the target property on the event to whatever you like? But the default filter is still useful:

function filter() {
  return !d3.event.sourceEvent.button;
}

Would it be weird to do this?

function target() {
  return d3.event.sourceEvent.button ? null : this;
}

Maybe context is a better name, since there’s already a concept of the event’s target, which is the DOM element.