From f39b7ae14a93bbe0c0632de567cea0bbb74bdb66 Mon Sep 17 00:00:00 2001
From: Uwe Oestermeier <u.oestermeier@iwm-tuebingen.de>
Date: Fri, 31 May 2019 15:32:13 +0200
Subject: [PATCH] Adding a return value in boundary condition.

---
 lib/scatter.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/scatter.js b/lib/scatter.js
index 168a6e2..0f2a1cb 100644
--- a/lib/scatter.js
+++ b/lib/scatter.js
@@ -410,7 +410,9 @@ export class AbstractScatter extends Throwable {
 
     keepOnStage(velocity, collision = 0.5) {
         let stagePolygon = this.containerPolygon
-        if (!stagePolygon) return
+        // UO: since keepOnStage is called in nextVelocity we need to 
+        // ensure a return value
+        if (!stagePolygon) return { x: 0, y: 0}
         let polygon = this.polygon
         let bounced = this.bouncing()
         if (bounced) {