From d48e6da62a0c9c7a9edecba36069a45d4d71d47b Mon Sep 17 00:00:00 2001
From: Jonathan Olson <olsonsjc@gmail.com>
Date: Wed, 1 Mar 2017 12:36:31 -0700
Subject: [PATCH] Don't step the simulation back in time. See
 https://github.com/phetsims/joist/issues/409

---
 js/Sim.js | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/js/Sim.js b/js/Sim.js
index a24f2c72..1f100d36 100644
--- a/js/Sim.js
+++ b/js/Sim.js
@@ -693,7 +693,11 @@ define( function( require ) {
 
       // Convert to seconds
       var dt = elapsedTimeMilliseconds / 1000.0;
-      this.stepSimulation( dt );
+
+      // Don't run the simulation on steps back in time (see https://github.com/phetsims/joist/issues/409).
+      if ( dt >= 0 ) {
+        this.stepSimulation( dt );
+      }
     },
 
     /**