Couple of tweaks to reset gravity.

Firstly, when the mouse exits the area the gravity coordinates are nullified.
Secondly, if the user switches to another window, the gravity coordinates are
nullified (otherwise the appropriate "mouseout" event may be missed).

examples/voroboids/voroboids.js

@@ -1,6 +1,6 @@
 var w = 960,
     h = 500,
-    mouse = [0, 0],
+    mouse = [null, null],
     fill = d3.scale.linear().domain([0, 1e4]).range(["brown", "steelblue"]);
 
 // Initialise boids.
@@ -16,6 +16,8 @@ var vertices = boids.map(function(boid) {
   return boid(boids);
 });
 
+d3.select(window).on("blur", nullGravity);
+
 var svg = d3.select("#vis")
   .append("svg:svg")
     .attr("width", w)
@@ -25,7 +27,8 @@ var svg = d3.select("#vis")
       var m = d3.svg.mouse(this);
       mouse[0] = m[0];
       mouse[1] = m[1];
-    });
+    })
+    .on("mouseout", nullGravity);
 
 svg.selectAll("path")
     .data(d3.geom.voronoi(vertices))
@@ -56,3 +59,7 @@ d3.timer(function() {
       .attr("d", function(d) { return "M" + d.join("L") + "Z"; })
       .style("fill", function(d) { return fill((d3.geom.polygon(d).area())); });
 });
+
+function nullGravity() {
+  mouse[0] = mouse[1] = null;
+}