Added middle click mouse down managment

src/canvas.class.js

@@ -288,6 +288,14 @@
      */
     fireRightClick: false,
 
+    /**
+     * Indicates if the canvas can fire middle click events
+     * @type Boolean
+     * @since 1.7.8
+     * @default
+     */
+    fireMiddleClick: false,
+
     /**
      * @private
      */

src/mixins/canvas_events.mixin.js

@@ -474,6 +474,14 @@
         return;
       }
 
+      var isMiddleClick  = 'which' in e ? e.which === 2 : e.button === 1;
+      if (isMiddleClick) {
+        if (this.fireMiddleClick) {
+          this._handleEvent(e, 'down', target ? target : null);
+        }
+        return;
+      }
+
       if (this.isDrawingMode) {
         this._onMouseDownInDrawingMode(e);
         return;

test/unit/canvas.js

@@ -1726,4 +1726,29 @@
     ok(typeof InheritedCanvasClass === 'function');
   });
 
+  test('mouse:down with different buttons', function() {
+    var clickCount = 0;
+    function mouseHandler() {
+      clickCount++;
+    }
+    canvas.on('mouse:down', mouseHandler);
+    canvas.fireMiddleClick = false;
+    canvas.fireRightClick = false;
+    canvas._currentTransform = false;
+    canvas.isDrawingMode = false;
+    canvas.__onMouseDown({ which: 1 });
+    equal(clickCount, 1, 'mouse down fired');
+    clickCount = 0;
+    canvas.__onMouseDown({ which: 3 });
+    equal(clickCount, 0, 'rightclick did not fire a mouse:down event');
+    canvas.fireRightClick = true;
+    canvas.__onMouseDown({ which: 3 });
+    equal(clickCount, 1, 'rightclick did fire a mouse:down event');
+    clickCount = 0;
+    canvas.__onMouseDown({ which: 2 });
+    equal(clickCount, 0, 'middleClick did not fire a mouse:down event');
+    canvas.fireMiddleClick = true;
+    canvas.__onMouseDown({ which: 2 });
+    equal(clickCount, 1, 'middleClick did fire a mouse:down event');
+  });
 })();