From b4dc4f85b0cf4f94aaee41e7dd41d87ed36ac945 Mon Sep 17 00:00:00 2001
From: mister-ben <git@misterben.me>
Date: Wed, 28 Jun 2017 07:36:06 +0100
Subject: [PATCH] fix: Use passive event listeners for touchstart/touchmove
 (#4440)

If passive event listening is supported, we should use it.

Fixes #4432.
---
 src/js/utils/events.js | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/src/js/utils/events.js b/src/js/utils/events.js
index 428ee32f78..281ce4884e 100644
--- a/src/js/utils/events.js
+++ b/src/js/utils/events.js
@@ -202,6 +202,33 @@ export function fixEvent(event) {
   return event;
 }
 
+/**
+ * Whether passive event listeners are supported
+ */
+let _supportsPassive = false;
+
+(function() {
+  try {
+    const opts = Object.defineProperty({}, 'passive', {
+      get() {
+        _supportsPassive = true;
+      }
+    });
+
+    window.addEventListener('test', null, opts);
+  } catch (e) {
+    // disregard
+  }
+})();
+
+/**
+ * Touch events Chrome expects to be passive
+ */
+const passiveEvents = [
+  'touchstart',
+  'touchmove'
+];
+
 /**
  * Add an event listener to element
  * It stores the handler function in a separate cache object
@@ -273,7 +300,13 @@ export function on(elem, type, fn) {
 
   if (data.handlers[type].length === 1) {
     if (elem.addEventListener) {
-      elem.addEventListener(type, data.dispatcher, false);
+      let options = false;
+
+      if (_supportsPassive &&
+        passiveEvents.indexOf(type) > -1) {
+        options = {passive: true};
+      }
+      elem.addEventListener(type, data.dispatcher, options);
     } else if (elem.attachEvent) {
       elem.attachEvent('on' + type, data.dispatcher);
     }