diff --git a/core/events.js b/core/events.js
index 3e196dc56..a4f0706a7 100644
--- a/core/events.js
+++ b/core/events.js
@@ -174,45 +174,37 @@ Blockly.Events.filter = function(queueIn, forward) {
     // Undo is merged in reverse order.
     queue.reverse();
   }
-  // Merge duplicates.  O(n^2), but n should be very small.
-  for (var i = 0, event1; event1 = queue[i]; i++) {
-    for (var j = i + 1, event2; event2 = queue[j]; j++) {
-      if (event1.type == event2.type &&
-          event1.blockId == event2.blockId &&
-          event1.workspaceId == event2.workspaceId) {
-        if (event1.type == Blockly.Events.MOVE) {
-          // Merge move events.
-          event1.newParentId = event2.newParentId;
-          event1.newInputName = event2.newInputName;
-          event1.newCoordinate = event2.newCoordinate;
-          queue.splice(j, 1);
-          j--;
-        } else if (event1.type == Blockly.Events.CHANGE &&
-            event1.element == event2.element &&
-            event1.name == event2.name) {
-          // Merge change events.
-          event1.newValue = event2.newValue;
-          queue.splice(j, 1);
-          j--;
-        } else if (event1.type == Blockly.Events.UI &&
-            event2.element == 'click' &&
-            (event1.element == 'commentOpen' ||
-             event1.element == 'mutatorOpen' ||
-             event1.element == 'warningOpen')) {
-          // Merge change events.
-          event1.newValue = event2.newValue;
-          queue.splice(j, 1);
-          j--;
-        }
+  var mergedQueue = [];
+  var hash = Object.create(null);
+  // Merge duplicates.
+  for (var i = 0, event; event = queue[i]; i++) {
+    if (!event.isNull()) {
+      var key = [event.type, event.blockId, event.workspaceId].join(' ');
+      var lastEvent = hash[key];
+      if (!lastEvent) {
+        hash[key] = event;
+        mergedQueue.push(event);
+      } else if (event.type == Blockly.Events.MOVE) {
+        // Merge move events.
+        lastEvent.newParentId = event.newParentId;
+        lastEvent.newInputName = event.newInputName;
+        lastEvent.newCoordinate = event.newCoordinate;
+      } else if (event.type == Blockly.Events.CHANGE &&
+          event.element == lastEvent.element &&
+          event.name == lastEvent.name) {
+        // Merge change events.
+        lastEvent.newValue = event.newValue;
+      } else if (event.type == Blockly.Events.UI &&
+          event.element == 'click' &&
+          (lastEvent.element == 'commentOpen' ||
+           lastEvent.element == 'mutatorOpen' ||
+           lastEvent.element == 'warningOpen')) {
+        // Merge click events.
+        lastEvent.newValue = event.newValue;
       }
     }
   }
-  // Remove null events.
-  for (var i = queue.length - 1; i >= 0; i--) {
-    if (queue[i].isNull()) {
-      queue.splice(i, 1);
-    }
-  }
+  queue = mergedQueue;
   if (!forward) {
     // Restore undo order.
     queue.reverse();
diff --git a/tests/jsunit/event_test.js b/tests/jsunit/event_test.js
index c019515e9..d5583089e 100644
--- a/tests/jsunit/event_test.js
+++ b/tests/jsunit/event_test.js
@@ -392,3 +392,136 @@ function test_varBackard_runForward() {
   checkVariableValues(workspace, 'name1', 'type1', 'id1');
   eventTest_tearDown();
 }
+
+function test_events_filter() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var events = [
+    new Blockly.Events.BlockCreate(block1),
+    new Blockly.Events.BlockMove(block1),
+    new Blockly.Events.BlockChange(block1, 'field', 'VAR', 'item', 'item1'),
+    new Blockly.Events.Ui(block1, 'click')
+  ];
+  var filteredEvents = Blockly.Events.filter(events, true);
+  assertEquals(4, filteredEvents.length);  // no event should have been removed.
+  // test that the order hasn't changed
+  assertTrue(filteredEvents[0] instanceof Blockly.Events.BlockCreate);
+  assertTrue(filteredEvents[1] instanceof Blockly.Events.BlockMove);
+  assertTrue(filteredEvents[2] instanceof Blockly.Events.BlockChange);
+  assertTrue(filteredEvents[3] instanceof Blockly.Events.Ui);
+}
+
+function test_events_filterForward() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var events = [
+    new Blockly.Events.BlockCreate(block1),
+  ];
+  helper_addMoveEvent(events, block1, 1, 1);
+  helper_addMoveEvent(events, block1, 2, 2);
+  helper_addMoveEvent(events, block1, 3, 3);
+  var filteredEvents = Blockly.Events.filter(events, true);
+  assertEquals(2, filteredEvents.length);  // duplicate moves should have been removed.
+  // test that the order hasn't changed
+  assertTrue(filteredEvents[0] instanceof Blockly.Events.BlockCreate);
+  assertTrue(filteredEvents[1] instanceof Blockly.Events.BlockMove);
+  assertEquals(3, filteredEvents[1].newCoordinate.x);
+  assertEquals(3, filteredEvents[1].newCoordinate.y);
+  eventTest_tearDownWithMockBlocks();
+}
+
+function test_events_filterBackward() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var events = [
+    new Blockly.Events.BlockCreate(block1),
+  ];
+  helper_addMoveEvent(events, block1, 1, 1);
+  helper_addMoveEvent(events, block1, 2, 2);
+  helper_addMoveEvent(events, block1, 3, 3);
+  var filteredEvents = Blockly.Events.filter(events, false);
+  assertEquals(2, filteredEvents.length);  // duplicate event should have been removed.
+  // test that the order hasn't changed
+  assertTrue(filteredEvents[0] instanceof Blockly.Events.BlockCreate);
+  assertTrue(filteredEvents[1] instanceof Blockly.Events.BlockMove);
+  assertEquals(1, filteredEvents[1].newCoordinate.x);
+  assertEquals(1, filteredEvents[1].newCoordinate.y);
+  eventTest_tearDownWithMockBlocks();
+}
+
+function test_events_filterDifferentBlocks() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var block2 = workspace.newBlock('field_variable_test_block', '2');
+  var events = [
+    new Blockly.Events.BlockCreate(block1),
+    new Blockly.Events.BlockMove(block1),
+    new Blockly.Events.BlockCreate(block2),
+    new Blockly.Events.BlockMove(block2)
+  ];
+  var filteredEvents = Blockly.Events.filter(events, true);
+  assertEquals(4, filteredEvents.length);  // no event should have been removed.
+  eventTest_tearDownWithMockBlocks();
+}
+
+function test_events_mergeMove() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var events = [];
+  helper_addMoveEvent(events, block1, 0, 0);
+  helper_addMoveEvent(events, block1, 1, 1);
+  var filteredEvents = Blockly.Events.filter(events, true);
+  assertEquals(1, filteredEvents.length);  // second move event merged into first
+  assertEquals(1, filteredEvents[0].newCoordinate.x);
+  assertEquals(1, filteredEvents[0].newCoordinate.y);
+  eventTest_tearDownWithMockBlocks();
+}
+
+function test_events_mergeChange() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var events = [
+    new Blockly.Events.Change(block1, 'field', 'VAR', 'item', 'item1'),
+    new Blockly.Events.Change(block1, 'field', 'VAR', 'item1', 'item2')
+  ];
+  var filteredEvents = Blockly.Events.filter(events, true);
+  assertEquals(1, filteredEvents.length);  // second change event merged into first
+  assertEquals('item', filteredEvents[0].oldValue);
+  assertEquals('item2', filteredEvents[0].newValue);
+  eventTest_tearDownWithMockBlocks();
+}
+
+function test_events_mergeUi() {
+  eventTest_setUpWithMockBlocks();
+  var block1 = workspace.newBlock('field_variable_test_block', '1');
+  var block2 = workspace.newBlock('field_variable_test_block', '2');
+  var block3 = workspace.newBlock('field_variable_test_block', '3');
+  var events = [
+    new Blockly.Events.Ui(block1, 'commentOpen', 'false', 'true'),
+    new Blockly.Events.Ui(block1, 'click', 'false', 'true'),
+    new Blockly.Events.Ui(block2, 'mutatorOpen', 'false', 'true'),
+    new Blockly.Events.Ui(block2, 'click', 'false', 'true'),
+    new Blockly.Events.Ui(block3, 'warningOpen', 'false', 'true'),
+    new Blockly.Events.Ui(block3, 'click', 'false', 'true')
+  ];
+  var filteredEvents = Blockly.Events.filter(events, true);
+  assertEquals(3, filteredEvents.length);  // click event merged into corresponding *Open event
+  assertEquals('commentOpen', filteredEvents[0].element);
+  assertEquals('mutatorOpen', filteredEvents[1].element);
+  assertEquals('warningOpen', filteredEvents[2].element);
+  eventTest_tearDownWithMockBlocks();
+}
+
+/**
+ * Helper function to simulate block move events.
+ *
+ * @param {!Array.<Blockly.Events.Abstract>} events a queue of events.
+ * @param {!Blockly.Block} block the block to be moved
+ * @param {number} newX new X coordinate of the block
+ * @param {number} newY new Y coordinate of the block
+ */
+function helper_addMoveEvent(events, block, newX, newY) {
+  events.push(new Blockly.Events.BlockMove(block));
+  block.xy_ = new goog.math.Coordinate(newX, newY);
+  events[events.length-1].recordNew();
+}