d3-drag 0.3.0 - "Cannot read property 'button' of null"
<p>I'm trying to use d3-drag with a canvas a such:</p>

<pre><code>select(canvas)
.call(
    drag()
    .container(canvas)
    .subject(partial(getNodeAtMouse, simulation, canvas))
    .on('start', someFunction))
</code></pre>

<p>However, I get the following error when I actually attempt to drag:</p>

<pre><code>Cannot read property 'button' of null
</code></pre>

<p>from the following line in d3-drag (d3 original source code)</p>

<pre><code>function defaultFilter() {
    return !d3Selection.event.button;
  }
</code></pre>

<p>If I remove that function (by specifying my own filter), I get the following error:</p>

<pre><code>Cannot read property 'sourceEvent' of null
</code></pre>

<p>In d3-selection (d3 original source code)</p>

<pre><code>function sourceEvent() {
    var current = exports.event, source;
    while (source = current.sourceEvent) current = source;
    return current;
  }
</code></pre>

<p>This makes me think that there is some bug between the expectations of d3-drag and d3-selection. Any ideas?</p>