Implement right click-rail handler

Resolve #568.

src/js/plugin/handler/click-rail.js

@@ -1,6 +1,5 @@
 'use strict';
 
-var _ = require('../../lib/helper');
 var instances = require('../instances');
 var updateGeometry = require('../update-geometry');
 var updateScroll = require('../update-scroll');
@@ -13,18 +12,10 @@ function bindClickRailHandler(element, i) {
 
   i.event.bind(i.scrollbarY, 'click', stopPropagation);
   i.event.bind(i.scrollbarYRail, 'click', function (e) {
-    var halfOfScrollbarLength = _.toInt(i.scrollbarYHeight / 2);
-    var positionTop = i.railYRatio * (e.pageY - window.pageYOffset - pageOffset(i.scrollbarYRail).top - halfOfScrollbarLength);
-    var maxPositionTop = i.railYRatio * (i.railYHeight - i.scrollbarYHeight);
-    var positionRatio = positionTop / maxPositionTop;
-
-    if (positionRatio < 0) {
-      positionRatio = 0;
-    } else if (positionRatio > 1) {
-      positionRatio = 1;
-    }
-
-    updateScroll(element, 'top', (i.contentHeight - i.containerHeight) * positionRatio);
+    var positionTop = e.pageY - window.pageYOffset - pageOffset(i.scrollbarYRail).top;
+    var direction = positionTop > i.scrollbarYTop ? 1 : -1;
+
+    updateScroll(element, 'top', element.scrollTop + direction * i.containerHeight);
     updateGeometry(element);
 
     e.stopPropagation();
@@ -32,18 +23,10 @@ function bindClickRailHandler(element, i) {
 
   i.event.bind(i.scrollbarX, 'click', stopPropagation);
   i.event.bind(i.scrollbarXRail, 'click', function (e) {
-    var halfOfScrollbarLength = _.toInt(i.scrollbarXWidth / 2);
-    var positionLeft = i.railXRatio * (e.pageX - window.pageXOffset - pageOffset(i.scrollbarXRail).left - halfOfScrollbarLength);
-    var maxPositionLeft = i.railXRatio * (i.railXWidth - i.scrollbarXWidth);
-    var positionRatio = positionLeft / maxPositionLeft;
-
-    if (positionRatio < 0) {
-      positionRatio = 0;
-    } else if (positionRatio > 1) {
-      positionRatio = 1;
-    }
-
-    updateScroll(element, 'left', ((i.contentWidth - i.containerWidth) * positionRatio) - i.negativeScrollAdjustment);
+    var positionLeft = e.pageX - window.pageXOffset - pageOffset(i.scrollbarXRail).left;
+    var direction = positionLeft > i.scrollbarXLeft ? 1 : -1;
+
+    updateScroll(element, 'left', element.scrollLeft + direction * i.containerWidth);
     updateGeometry(element);
 
     e.stopPropagation();