Normalize mouse wheel deltas (#3911)

* Normalize mouse wheel deltas for #3792

* include support for IE6/7/8

lib/timeline/Core.js

@@ -176,6 +176,11 @@ Core.prototype._create = function (container) {
    * @param {WheelEvent} event
    */
   function onMouseWheel(event) {
+
+    // Reasonable default wheel deltas
+    const LINE_HEIGHT = 40;
+    const PAGE_HEIGHT = 800;
+
     if (this.isActive()) {
       this.emit('mousewheel', event);
     }
@@ -204,6 +209,17 @@ Core.prototype._create = function (container) {
       deltaX = event.deltaX;
     }
 
+    // Normalize deltas
+    if (event.deltaMode) {
+      if (event.deltaMode === 1) {   // delta in LINE units
+        deltaX *= LINE_HEIGHT;
+        deltaY *= LINE_HEIGHT;
+      } else {                       // delta in PAGE units
+        deltaX *= LINE_HEIGHT;
+        deltaY *= PAGE_HEIGHT;
+      }
+    }
+
     // Prevent scrolling when zooming (no zoom key, or pressing zoom key)
     if (!this.options.zoomKey || event[this.options.zoomKey]) return;
 
@@ -241,11 +257,12 @@ Core.prototype._create = function (container) {
     }
   }
 
+  // Add modern wheel event listener
   if (this.dom.centerContainer.addEventListener) {
-    // IE9, Chrome, Safari, Opera
-    this.dom.centerContainer.addEventListener("mousewheel", onMouseWheel.bind(this), false);
-    // Firefox
-    this.dom.centerContainer.addEventListener("DOMMouseScroll", onMouseWheel.bind(this), false);
+    const wheel = "onwheel" in document.createElement("div") ? "wheel" : // Modern browsers support "wheel"
+      document.onmousewheel !== undefined ? "mousewheel" : // Webkit and IE support at least "mousewheel"
+      "DOMMouseScroll"; // Older Firefox versions like "DOMMouseScroll"
+    this.dom.centerContainer.addEventListener(wheel, onMouseWheel.bind(this), false);
   } else {
     // IE 6/7/8
     this.dom.centerContainer.attachEvent("onmousewheel", onMouseWheel.bind(this));