Scroll the attached view when wheel spinned on top of a scroll button.

ui/file_manager/file_manager/foreground/js/ui/scrollbar.js

Index: ui/file_manager/file_manager/foreground/js/ui/scrollbar.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/scrollbar.js b/ui/file_manager/file_manager/foreground/js/ui/scrollbar.js
index 479c8cc72f0dd98e0a3ee89e781e91198270b1c1..041fb434fca51100ef311c0c62f384e03f9c5f7c 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/scrollbar.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/scrollbar.js
@@ -51,6 +51,7 @@ ScrollBar.prototype.decorate = function() {
 
   this.button_.addEventListener('mousedown',
                                 this.onButtonPressed_.bind(this));
+  this.button_.addEventListener('wheel', this.onWheel_.bind(this));
   window.addEventListener('mouseup', this.onMouseUp_.bind(this));
   window.addEventListener('mousemove', this.onMouseMove_.bind(this));
 };
@@ -86,6 +87,27 @@ ScrollBar.prototype.attachToView = function(view) {
 };
 
 /**
+ * Handles scroll by a mouse wheel.
+ *
+ * @param {Event} event Mouse event.
+ * @private
+ */
+ScrollBar.prototype.onWheel_ = function(event) {
+  var scrollPosition = this.view_.scrollTop + event.deltaY;
+  // Ensure the scrollbar is in the view.
+  var scrollBottom =
+      Math.max(this.view_.scrollHeight - this.view_.clientHeight, 0);
+  scrollPosition = Math.max(Math.min(scrollPosition, scrollBottom), 0);
+
+  // TODO(yamaguchi): Invoke native scroll instead of setting scrollTop.
+  //     This implementation will bypass the smooth scroll animation seen with

[fukino, 2016/07/11 10:19:12]

nit: You don't need to indent here.

[yamaguchi, 2016/07/11 10:23:07]

Done.

+  //     native scroll.
+  this.scrollTop_ = scrollPosition;
+  this.view_.scrollTop = scrollPosition;
+  this.redraw_();
+}
+
+/**
  * Scroll handler.
  * @private
  */