Only notify when extent changes.

This avoids unnecessary work and some infinite loops.

d3.js

@@ -7975,13 +7975,20 @@ d3 = function() {
             });
           });
         });
-      } else {
+      } else if (extent0 !== extent) {
         extent0 = extent;
         g.each(function() {
-          event.of(this, arguments)({
+          var event_ = event.of(this, arguments);
+          event_({
+            type: "brushstart"
+          });
+          event_({
             type: "brush",
             mode: "resize"
           });
+          event_({
+            type: "brushend"
+          });
         });
       }
     }
@@ -8128,7 +8135,7 @@ d3 = function() {
       return brush;
     };
     brush.extent = function(z) {
-      var x0, x1, y0, y1, t;
+      var x0, x1, y0, y1, t, dirty;
       if (!arguments.length) {
         z = extentDomain || extent;
         if (x) {
@@ -8156,6 +8163,7 @@ d3 = function() {
         extentDomain[0][0] = x0, extentDomain[1][0] = x1;
         if (x.invert) x0 = x(x0), x1 = x(x1);
         if (x1 < x0) t = x0, x0 = x1, x1 = t;
+        dirty |= x0 ^ extent[0][0] | x1 ^ extent[1][0];
       }
       if (y) {
         y0 = z[0], y1 = z[1];
@@ -8163,13 +8171,14 @@ d3 = function() {
         extentDomain[0][1] = y0, extentDomain[1][1] = y1;
         if (y.invert) y0 = y(y0), y1 = y(y1);
         if (y1 < y0) t = y0, y0 = y1, y1 = t;
+        dirty |= y0 ^ extent[0][1] | y1 ^ extent[1][1];
       }
-      extent = [ [ x0 | 0, y0 | 0 ], [ x1 | 0, y1 | 0 ] ];
+      if (dirty) extent = [ [ x0 | 0, y0 | 0 ], [ x1 | 0, y1 | 0 ] ];
       return brush;
     };
     brush.clear = function() {
       extentDomain = null;
-      extent = [ [ 0, 0 ], [ 0, 0 ] ];
+      if (!brush.empty()) extent = [ [ 0, 0 ], [ 0, 0 ] ];
       return brush;
     };
     brush.empty = function() {

src/svg/brush.js

@@ -96,10 +96,13 @@ d3.svg.brush = function() {
             })
             .each("end.brush", function() { event_({type: "brushend"}); });
       });
-    } else {
+    } else if (extent0 !== extent) {
       extent0 = extent;
       g.each(function() {
-        event.of(this, arguments)({type: "brush", mode: "resize"});
+        var event_ = event.of(this, arguments);
+        event_({type: "brushstart"});
+        event_({type: "brush", mode: "resize"});
+        event_({type: "brushend"});
       });
     }
   }
@@ -328,7 +331,7 @@ d3.svg.brush = function() {
   };
 
   brush.extent = function(z) {
-    var x0, x1, y0, y1, t;
+    var x0, x1, y0, y1, t, dirty;
 
     // Invert the pixel extent to data-space.
     if (!arguments.length) {
@@ -360,6 +363,7 @@ d3.svg.brush = function() {
       extentDomain[0][0] = x0, extentDomain[1][0] = x1;
       if (x.invert) x0 = x(x0), x1 = x(x1);
       if (x1 < x0) t = x0, x0 = x1, x1 = t;
+      dirty |= x0 ^ extent[0][0] | x1 ^ extent[1][0];
     }
     if (y) {
       y0 = z[0], y1 = z[1];
@@ -367,15 +371,16 @@ d3.svg.brush = function() {
       extentDomain[0][1] = y0, extentDomain[1][1] = y1;
       if (y.invert) y0 = y(y0), y1 = y(y1);
       if (y1 < y0) t = y0, y0 = y1, y1 = t;
+      dirty |= y0 ^ extent[0][1] | y1 ^ extent[1][1];
     }
 
-    extent = [[x0 | 0, y0 | 0], [x1 | 0, y1 | 0]]; // copy-on-write
+    if (dirty) extent = [[x0 | 0, y0 | 0], [x1 | 0, y1 | 0]]; // copy-on-write
     return brush;
   };
 
   brush.clear = function() {
     extentDomain = null;
-    extent = [[0, 0], [0, 0]]; // copy-on-write
+    if (!brush.empty()) extent = [[0, 0], [0, 0]]; // copy-on-write
     return brush;
   };