From 4ae34599e032b7bf03b991d70003043771738b1c Mon Sep 17 00:00:00 2001
From: rachel-fenichel <fenichel@google.com>
Date: Fri, 11 Mar 2016 17:38:59 -0800
Subject: [PATCH] Don't connect a block with no next connection if that would
 force a bump.

---
 core/connection.js              |  8 ++++++++
 tests/jsunit/connection_test.js | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/core/connection.js b/core/connection.js
index 4db07585f..1e38fe42c 100644
--- a/core/connection.js
+++ b/core/connection.js
@@ -373,6 +373,14 @@ Blockly.Connection.prototype.isConnectionAllowed = function(candidate,
     return false;
   }
 
+  // Don't let a block with no next connection bump other blocks out of the
+  // stack.
+  if (this.type == Blockly.PREVIOUS_STATEMENT &&
+      candidate.targetConnection &&
+      !this.sourceBlock_.nextConnection) {
+    return false;
+  }
+
   // Don't let blocks try to connect to themselves or ones they nest.
   var targetSourceBlock = candidate.sourceBlock_;
   var sourceBlock = this.sourceBlock_;
diff --git a/tests/jsunit/connection_test.js b/tests/jsunit/connection_test.js
index e0b91efee..c9ad00dae 100644
--- a/tests/jsunit/connection_test.js
+++ b/tests/jsunit/connection_test.js
@@ -279,6 +279,25 @@ function test_isConnectionAllowed() {
   assertFalse(one.isConnectionAllowed(two, 1000.0));
 }
 
+function test_isConnectionAllowed_NoNext() {
+  var sharedWorkspace = {};
+  var one = helper_createConnection(0, 0, Blockly.NEXT_STATEMENT);
+  one.sourceBlock_ = helper_makeSourceBlock(sharedWorkspace);
+  one.sourceBlock_.nextConnection = one;
+
+  var two = helper_createConnection(0, 0, Blockly.PREVIOUS_STATEMENT);
+  two.sourceBlock_ = helper_makeSourceBlock(sharedWorkspace);
+
+  assertTrue(two.isConnectionAllowed(one, 1000.0));
+
+  var three = helper_createConnection(0, 0, Blockly.PREVIOUS_STATEMENT);
+  three.sourceBlock_ = helper_makeSourceBlock(sharedWorkspace);
+  three.sourceBlock_.previousConnection = three;
+  Blockly.Connection.connectReciprocally_(one, three);
+
+  assertFalse(two.isConnectionAllowed(one, 1000.0));
+}
+
 function testCheckConnection_Okay() {
   connectionTest_setUp();
   previous.checkConnection_(next);