consider the last block on the stack when looking for places to attach a dragging stack.

core/block.js

@@ -278,6 +278,25 @@ Blockly.Block.prototype.getConnections_ = function(all) {
   return myConnections;
 };
 
+/**
+ * Walks down a stack of blocks and finds the last next connection on the stack.
+ * @return {Blockly.Connection} The last next connection on the stack, or null.
+ * @private
+ */
+Blockly.Block.prototype.lastConnectionInStack_ = function() {
+  var nextConnection = this.nextConnection;
+  while (nextConnection) {
+    var nextBlock = nextConnection.targetBlock();
+    if (!nextBlock) {
+      // Found a next connection with nothing on the other side.
+      return nextConnection;
+    }
+    nextConnection = nextBlock.nextConnection;
+  }
+  // Ran out of next connections.
+  return null;
+};
+
 /**
  * Bump unconnected blocks out of alignment.  Two blocks which aren't actually
  * connected should not coincidentally line up on screen.

core/block_svg.js

@@ -822,6 +822,11 @@ Blockly.BlockSvg.prototype.onMouseMove_ = function(e) {
     // Check to see if any of this block's connections are within range of
     // another block's connection.
     var myConnections = this.getConnections_(false);
+    // Also check the last connection on this stack
+    var lastOnStack = this.lastConnectionInStack_();
+    if (lastOnStack && lastOnStack != this.nextConnection) {
+      myConnections.push(lastOnStack);
+    }
     var closestConnection = null;
     var localConnection = null;
     var radiusConnection = Blockly.SNAP_RADIUS;