diff --git a/src/__celerity_resource_map__.php b/src/__celerity_resource_map__.php
index 467e03734d..ce9285077a 100644
--- a/src/__celerity_resource_map__.php
+++ b/src/__celerity_resource_map__.php
@@ -1676,6 +1676,19 @@ celerity_register_resource_map(array(
     ),
     'disk' => '/rsrc/js/application/core/behavior-tooltip.js',
   ),
+  'javelin-behavior-phabricator-transaction-comment-form' =>
+  array(
+    'uri' => '/res/bdc362ee/rsrc/js/application/transactions/behavior-transaction-comment-form.js',
+    'type' => 'js',
+    'requires' =>
+    array(
+      0 => 'javelin-behavior',
+      1 => 'javelin-dom',
+      2 => 'javelin-util',
+      3 => 'phabricator-shaped-request',
+    ),
+    'disk' => '/rsrc/js/application/transactions/behavior-transaction-comment-form.js',
+  ),
   'javelin-behavior-phabricator-transaction-list' =>
   array(
     'uri' => '/res/212f97ba/rsrc/js/application/transactions/behavior-transaction-list.js',
@@ -2847,7 +2860,7 @@ celerity_register_resource_map(array(
   ),
   'phabricator-timeline-view-css' =>
   array(
-    'uri' => '/res/1846f7d5/rsrc/css/layout/phabricator-timeline-view.css',
+    'uri' => '/res/9cbeb7f0/rsrc/css/layout/phabricator-timeline-view.css',
     'type' => 'css',
     'requires' =>
     array(
diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 9f0466cc20..fe90436abf 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -613,6 +613,7 @@ phutil_register_library_map(array(
     'PhabricatorApplicationTransactionCommentEditor' => 'applications/transactions/editor/PhabricatorApplicationTransactionCommentEditor.php',
     'PhabricatorApplicationTransactionCommentHistoryController' => 'applications/transactions/controller/PhabricatorApplicationTransactionCommentHistoryController.php',
     'PhabricatorApplicationTransactionCommentQuery' => 'applications/transactions/query/PhabricatorApplicationTransactionCommentQuery.php',
+    'PhabricatorApplicationTransactionCommentView' => 'applications/transactions/view/PhabricatorApplicationTransactionCommentView.php',
     'PhabricatorApplicationTransactionController' => 'applications/transactions/controller/PhabricatorApplicationTransactionController.php',
     'PhabricatorApplicationTransactionEditor' => 'applications/transactions/editor/PhabricatorApplicationTransactionEditor.php',
     'PhabricatorApplicationTransactionFeedStory' => 'applications/transactions/feed/PhabricatorApplicationTransactionFeedStory.php',
@@ -1904,6 +1905,7 @@ phutil_register_library_map(array(
     'PhabricatorApplicationTransactionCommentEditor' => 'PhabricatorEditor',
     'PhabricatorApplicationTransactionCommentHistoryController' => 'PhabricatorApplicationTransactionController',
     'PhabricatorApplicationTransactionCommentQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
+    'PhabricatorApplicationTransactionCommentView' => 'AphrontView',
     'PhabricatorApplicationTransactionController' => 'PhabricatorController',
     'PhabricatorApplicationTransactionEditor' => 'PhabricatorEditor',
     'PhabricatorApplicationTransactionFeedStory' => 'PhabricatorFeedStory',
diff --git a/src/aphront/AphrontRequest.php b/src/aphront/AphrontRequest.php
index aba20d6747..762d2394cd 100644
--- a/src/aphront/AphrontRequest.php
+++ b/src/aphront/AphrontRequest.php
@@ -18,6 +18,7 @@ final class AphrontRequest {
   const TYPE_CONDUIT = '__conduit__';
   const TYPE_WORKFLOW = '__wflow__';
   const TYPE_CONTINUE = '__continue__';
+  const TYPE_PREVIEW = '__preview__';
 
   private $host;
   private $path;
@@ -339,6 +340,10 @@ final class AphrontRequest {
     return $this->isFormPost() && $this->getStr('__continue__');
   }
 
+  public function isPreviewRequest() {
+    return $this->isFormPost() && $this->getStr('__preview__');
+  }
+
   /**
    * Get application request parameters in a flattened form suitable for
    * inclusion in an HTTP request, excluding parameters with special meanings.
diff --git a/src/applications/macro/controller/PhabricatorMacroCommentController.php b/src/applications/macro/controller/PhabricatorMacroCommentController.php
index 9b4fe7763d..a1ba47212a 100644
--- a/src/applications/macro/controller/PhabricatorMacroCommentController.php
+++ b/src/applications/macro/controller/PhabricatorMacroCommentController.php
@@ -22,10 +22,11 @@ final class PhabricatorMacroCommentController
       return new Aphront404Response();
     }
 
+    $is_preview = $request->isPreviewRequest();
+
     $view_uri = $this->getApplicationURI('/view/'.$macro->getID().'/');
 
     $xactions = array();
-
     $xactions[] = id(new PhabricatorMacroTransaction())
       ->setTransactionType(PhabricatorTransactions::TYPE_COMMENT)
       ->attachComment(
@@ -40,7 +41,8 @@ final class PhabricatorMacroCommentController
           PhabricatorContentSource::SOURCE_WEB,
           array(
             'ip' => $request->getRemoteAddr(),
-          )));
+          )))
+      ->setIsPreview($is_preview);
 
     try {
       $xactions = $editor->applyTransactions($macro, $xactions);
@@ -54,6 +56,7 @@ final class PhabricatorMacroCommentController
       return id(new PhabricatorApplicationTransactionResponse())
         ->setViewer($user)
         ->setTransactions($xactions)
+        ->setIsPreview($is_preview)
         ->setAnchorOffset($request->getStr('anchor'));
     } else {
       return id(new AphrontRedirectResponse())
diff --git a/src/applications/macro/controller/PhabricatorMacroViewController.php b/src/applications/macro/controller/PhabricatorMacroViewController.php
index 30b417101a..547ba80c30 100644
--- a/src/applications/macro/controller/PhabricatorMacroViewController.php
+++ b/src/applications/macro/controller/PhabricatorMacroViewController.php
@@ -79,23 +79,14 @@ final class PhabricatorMacroViewController
           ? pht('Add Comment')
           : pht('Grovel in Awe'));
 
-    $add_comment_form = id(new AphrontFormView())
-      ->setWorkflow(true)
-      ->setFlexible(true)
-      ->addSigil('transaction-append')
-      ->setAction($this->getApplicationURI('/comment/'.$macro->getID().'/'))
+    $submit_button_name = $is_serious
+      ? pht('Add Comment')
+      : pht('Lavish Praise');
+
+    $add_comment_form = id(new PhabricatorApplicationTransactionCommentView())
       ->setUser($user)
-      ->appendChild(
-        id(new PhabricatorRemarkupControl())
-          ->setUser($user)
-          ->setLabel('Comment')
-          ->setName('comment'))
-      ->appendChild(
-        id(new AphrontFormSubmitControl())
-          ->setValue(
-            $is_serious
-              ? pht('Add Comment')
-              : pht('Lavish Praise')));
+      ->setAction($this->getApplicationURI('/comment/'.$macro->getID().'/'))
+      ->setSubmitButtonName($submit_button_name);
 
     return $this->buildApplicationPage(
       array(
diff --git a/src/applications/pholio/controller/PholioMockCommentController.php b/src/applications/pholio/controller/PholioMockCommentController.php
index cf1537f4e4..8ee3c8d218 100644
--- a/src/applications/pholio/controller/PholioMockCommentController.php
+++ b/src/applications/pholio/controller/PholioMockCommentController.php
@@ -15,6 +15,10 @@ final class PholioMockCommentController extends PholioController {
     $request = $this->getRequest();
     $user = $request->getUser();
 
+    if (!$request->isFormPost()) {
+      return new Aphront400Response();
+    }
+
     $mock = id(new PholioMockQuery())
       ->setViewer($user)
       ->withIDs(array($this->id))
@@ -24,6 +28,8 @@ final class PholioMockCommentController extends PholioController {
       return new Aphront404Response();
     }
 
+    $is_preview = $request->isPreviewRequest();
+
     $mock_uri = '/M'.$mock->getID();
 
     $comment = $request->getStr('comment');
@@ -44,7 +50,8 @@ final class PholioMockCommentController extends PholioController {
     $editor = id(new PholioMockEditor())
       ->setActor($user)
       ->setContentSource($content_source)
-      ->setContinueOnException($request->isContinueRequest());
+      ->setContinueOnNoEffect($request->isContinueRequest())
+      ->setIsPreview($is_preview);
 
     try {
       $xactions = $editor->applyTransactions($mock, $xactions);
@@ -58,6 +65,7 @@ final class PholioMockCommentController extends PholioController {
       return id(new PhabricatorApplicationTransactionResponse())
         ->setViewer($user)
         ->setTransactions($xactions)
+        ->setIsPreview($is_preview)
         ->setAnchorOffset($request->getStr('anchor'));
     } else {
       return id(new AphrontRedirectResponse())->setURI($mock_uri);
diff --git a/src/applications/pholio/controller/PholioMockEditController.php b/src/applications/pholio/controller/PholioMockEditController.php
index 10d66fc652..cfc1da1022 100644
--- a/src/applications/pholio/controller/PholioMockEditController.php
+++ b/src/applications/pholio/controller/PholioMockEditController.php
@@ -120,7 +120,7 @@ final class PholioMockEditController extends PholioController {
         $mock->openTransaction();
           $editor = id(new PholioMockEditor())
             ->setContentSource($content_source)
-            ->setContinueOnException(true)
+            ->setContinueOnNoEffect(true)
             ->setActor($user);
 
           $xactions = $editor->applyTransactions($mock, $xactions);
diff --git a/src/applications/pholio/controller/PholioMockViewController.php b/src/applications/pholio/controller/PholioMockViewController.php
index 07b870b55c..9f23d46e1f 100644
--- a/src/applications/pholio/controller/PholioMockViewController.php
+++ b/src/applications/pholio/controller/PholioMockViewController.php
@@ -65,7 +65,7 @@ final class PholioMockViewController extends PholioController {
         'Carousel Goes Here</h1>';
 
     $xaction_view = id(new PhabricatorApplicationTransactionView())
-      ->setViewer($this->getRequest()->getUser())
+      ->setUser($this->getRequest()->getUser())
       ->setTransactions($xactions)
       ->setMarkupEngine($engine);
 
@@ -168,24 +168,14 @@ final class PholioMockViewController extends PholioController {
     $header = id(new PhabricatorHeaderView())
       ->setHeader($title);
 
-    $action = $is_serious
+    $button_name = $is_serious
       ? pht('Add Comment')
       : pht('Answer The Call');
 
-    $form = id(new AphrontFormView())
+    $form = id(new PhabricatorApplicationTransactionCommentView())
       ->setUser($user)
-      ->addSigil('transaction-append')
-      ->setAction($this->getApplicationURI('/comment/'.$mock->getID().'/'))
-      ->setWorkflow(true)
-      ->setFlexible(true)
-      ->appendChild(
-        id(new PhabricatorRemarkupControl())
-          ->setName('comment')
-          ->setLabel(pht('Comment'))
-          ->setUser($user))
-      ->appendChild(
-        id(new AphrontFormSubmitControl())
-          ->setValue($action));
+      ->setSubmitButtonName($button_name)
+      ->setAction($this->getApplicationURI('/comment/'.$mock->getID().'/'));
 
     return array(
       $header,
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
index 5edd417cfe..3d99a4aef0 100644
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -15,6 +15,7 @@ abstract class PhabricatorApplicationTransactionEditor
   private $mentionedPHIDs;
   private $continueOnNoEffect;
 
+  private $isPreview;
 
   /**
    * When the editor tries to apply transactions that have no effect, should
@@ -46,6 +47,15 @@ abstract class PhabricatorApplicationTransactionEditor
     return $this->mentionedPHIDs;
   }
 
+  public function setIsPreview($is_preview) {
+    $this->isPreview = $is_preview;
+    return $this;
+  }
+
+  public function getIsPreview() {
+    return $this->isPreview;
+  }
+
   public function getTransactionTypes() {
     $types = array(
       PhabricatorTransactions::TYPE_COMMENT,
@@ -225,11 +235,16 @@ abstract class PhabricatorApplicationTransactionEditor
     $xactions = $this->filterTransactions($object, $xactions);
 
     if (!$xactions) {
-      return $this;
+      return array();
     }
 
     $xactions = $this->sortTransactions($xactions);
 
+    if ($this->getIsPreview()) {
+      $this->loadHandles($xactions);
+      return $xactions;
+    }
+
     $comment_editor = id(new PhabricatorApplicationTransactionCommentEditor())
       ->setActor($actor)
       ->setContentSource($this->getContentSource());
@@ -256,10 +271,8 @@ abstract class PhabricatorApplicationTransactionEditor
       }
     $object->saveTransaction();
 
-
     $this->loadHandles($xactions);
 
-
     $mail = null;
     if ($this->supportsMail()) {
       $mail = $this->sendMail($object, $xactions);
@@ -285,8 +298,8 @@ abstract class PhabricatorApplicationTransactionEditor
 
   private function loadHandles(array $xactions) {
     $phids = array();
-    foreach ($xactions as $xaction) {
-      $phids[$xaction->getPHID()] = $xaction->getRequiredHandlePHIDs();
+    foreach ($xactions as $key => $xaction) {
+      $phids[$key] = $xaction->getRequiredHandlePHIDs();
     }
     $handles = array();
     $merged = array_mergev($phids);
@@ -295,11 +308,8 @@ abstract class PhabricatorApplicationTransactionEditor
         ->setViewer($this->requireActor())
         ->loadHandles();
     }
-    foreach ($xactions as $xaction) {
-      $xaction->setHandles(
-        array_select_keys(
-          $handles,
-          $phids[$xaction->getPHID()]));
+    foreach ($xactions as $key => $xaction) {
+      $xaction->setHandles(array_select_keys($handles, $phids[$key]));
     }
   }
 
@@ -587,13 +597,18 @@ abstract class PhabricatorApplicationTransactionEditor
       return $xactions;
     }
 
-    if (!$this->getContinueOnNoEffect()) {
+    if (!$this->getContinueOnNoEffect() && !$this->getIsPreview()) {
       throw new PhabricatorApplicationTransactionNoEffectException(
         $no_effect,
         $any_effect,
         $has_comment);
     }
 
+    if (!$any_effect && !$has_comment) {
+      // If we only have empty comment transactions, just drop them all.
+      return array();
+    }
+
     foreach ($no_effect as $key => $xaction) {
       if ($xaction->getComment()) {
         $xaction->setTransactionType($type_comment);
diff --git a/src/applications/transactions/response/PhabricatorApplicationTransactionResponse.php b/src/applications/transactions/response/PhabricatorApplicationTransactionResponse.php
index cc5eede13d..1ddb9733eb 100644
--- a/src/applications/transactions/response/PhabricatorApplicationTransactionResponse.php
+++ b/src/applications/transactions/response/PhabricatorApplicationTransactionResponse.php
@@ -6,6 +6,7 @@ final class PhabricatorApplicationTransactionResponse
   private $viewer;
   private $transactions;
   private $anchorOffset;
+  private $isPreview;
 
   protected function buildProxy() {
     return new AphrontAjaxResponse();
@@ -40,16 +41,26 @@ final class PhabricatorApplicationTransactionResponse
     return $this->viewer;
   }
 
+  public function setIsPreview($is_preview) {
+    $this->isPreview = $is_preview;
+    return $this;
+  }
+
   public function reduceProxyResponse() {
     $view = id(new PhabricatorApplicationTransactionView())
-      ->setViewer($this->getViewer())
-      ->setTransactions($this->getTransactions());
+      ->setUser($this->getViewer())
+      ->setTransactions($this->getTransactions())
+      ->setIsPreview($this->isPreview);
 
     if ($this->getAnchorOffset()) {
       $view->setAnchorOffset($this->getAnchorOffset());
     }
 
-    $xactions = mpull($view->buildEvents(), 'render', 'getTransactionPHID');
+    if ($this->isPreview) {
+      $xactions = mpull($view->buildEvents(), 'render');
+    } else {
+      $xactions = mpull($view->buildEvents(), 'render', 'getTransactionPHID');
+    }
 
     $content = array(
       'xactions' => $xactions,
diff --git a/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php b/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php
new file mode 100644
index 0000000000..26e20c853c
--- /dev/null
+++ b/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php
@@ -0,0 +1,147 @@
+<?php
+
+/**
+ * @concrete-extensible
+ */
+class PhabricatorApplicationTransactionCommentView extends AphrontView {
+
+  private $submitButtonName;
+  private $action;
+
+  private $previewPanelID;
+  private $previewTimelineID;
+  private $previewToggleID;
+  private $formID;
+  private $statusID;
+
+  public function setSubmitButtonName($submit_button_name) {
+    $this->submitButtonName = $submit_button_name;
+    return $this;
+  }
+
+  public function getSubmitButtonName() {
+    return $this->submitButtonName;
+  }
+
+  public function setAction($action) {
+    $this->action = $action;
+    return $this;
+  }
+
+  public function getAction() {
+    return $this->action;
+  }
+
+  public function render() {
+
+    $data = array();
+
+    $comment = $this->renderCommentPanel();
+
+    $preview = $this->renderPreviewPanel();
+
+    Javelin::initBehavior(
+      'phabricator-transaction-comment-form',
+      array(
+        'formID'        => $this->getFormID(),
+        'timelineID'    => $this->getPreviewTimelineID(),
+        'panelID'       => $this->getPreviewPanelID(),
+        'statusID'      => $this->getStatusID(),
+
+        'loadingString' => pht('Loading Preview...'),
+        'savingString'  => pht('Saving Draft...'),
+        'draftString'   => pht('Saved Draft'),
+
+        'actionURI'     => $this->getAction(),
+      ));
+
+    return self::renderSingleView(
+      array(
+        $comment,
+        $preview,
+      ));
+  }
+
+  private function renderCommentPanel() {
+    $status = phutil_render_tag(
+      'div',
+      array(
+        'id' => $this->getStatusID(),
+      ),
+      '');
+
+    return id(new AphrontFormView())
+      ->setUser($this->getUser())
+      ->setFlexible(true)
+      ->addSigil('transaction-append')
+      ->setWorkflow(true)
+      ->setAction($this->getAction())
+      ->setID($this->getFormID())
+      ->appendChild(
+        id(new PhabricatorRemarkupControl())
+          ->setName('comment')
+          ->setLabel(pht('Comment'))
+          ->setUser($this->getUser()))
+      ->appendChild(
+        id(new AphrontFormSubmitControl())
+          ->setValue($this->getSubmitButtonName()))
+      ->appendChild(
+        id(new AphrontFormMarkupControl())
+          ->setValue($status));
+  }
+
+  private function renderPreviewPanel() {
+
+    $preview = id(new PhabricatorTimelineView())
+      ->setID($this->getPreviewTimelineID());
+
+    $header = phutil_render_tag(
+      'div',
+      array(
+        'class' => 'phabricator-timeline-preview-header',
+      ),
+      phutil_escape_html(pht('Preview')));
+
+    return phutil_render_tag(
+      'div',
+      array(
+        'id'    => $this->getPreviewPanelID(),
+        'style' => 'display: none',
+      ),
+      self::renderSingleView(
+        array(
+          $header,
+          $preview,
+        )));
+  }
+
+  private function getPreviewPanelID() {
+    if (!$this->previewPanelID) {
+      $this->previewPanelID = celerity_generate_unique_node_id();
+    }
+    return $this->previewPanelID;
+  }
+
+  private function getPreviewTimelineID() {
+    if (!$this->previewTimelineID) {
+      $this->previewTimelineID = celerity_generate_unique_node_id();
+    }
+    return $this->previewTimelineID;
+  }
+
+  private function getFormID() {
+    if (!$this->formID) {
+      $this->formID = celerity_generate_unique_node_id();
+    }
+    return $this->formID;
+  }
+
+  private function getStatusID() {
+    if (!$this->statusID) {
+      $this->statusID = celerity_generate_unique_node_id();
+    }
+    return $this->statusID;
+  }
+
+}
+
diff --git a/src/applications/transactions/view/PhabricatorApplicationTransactionView.php b/src/applications/transactions/view/PhabricatorApplicationTransactionView.php
index df4bfda588..07bcbafd47 100644
--- a/src/applications/transactions/view/PhabricatorApplicationTransactionView.php
+++ b/src/applications/transactions/view/PhabricatorApplicationTransactionView.php
@@ -5,11 +5,16 @@
  */
 class PhabricatorApplicationTransactionView extends AphrontView {
 
-  private $viewer;
   private $transactions;
   private $engine;
   private $anchorOffset = 1;
   private $showEditActions = true;
+  private $isPreview;
+
+  public function setIsPreview($is_preview) {
+    $this->isPreview = $is_preview;
+    return $this;
+  }
 
   public function setShowEditActions($show_edit_actions) {
     $this->showEditActions = $show_edit_actions;
@@ -36,16 +41,11 @@ class PhabricatorApplicationTransactionView extends AphrontView {
     return $this;
   }
 
-  public function setViewer(PhabricatorUser $viewer) {
-    $this->viewer = $viewer;
-    return $this;
-  }
-
   public function buildEvents() {
     $field = PhabricatorApplicationTransactionComment::MARKUP_FIELD_COMMENT;
     $engine = $this->getOrBuildEngine();
 
-    $viewer = $this->viewer;
+    $user = $this->getUser();
 
     $anchor = $this->anchorOffset;
     $events = array();
@@ -55,22 +55,29 @@ class PhabricatorApplicationTransactionView extends AphrontView {
       }
 
       $event = id(new PhabricatorTimelineEventView())
-        ->setViewer($viewer)
+        ->setUser($user)
         ->setTransactionPHID($xaction->getPHID())
         ->setUserHandle($xaction->getHandle($xaction->getAuthorPHID()))
         ->setIcon($xaction->getIcon())
         ->setColor($xaction->getColor())
-        ->setTitle($xaction->getTitle())
-        ->setDateCreated($xaction->getDateCreated())
-        ->setContentSource($xaction->getContentSource())
-        ->setAnchor($anchor);
+        ->setTitle($xaction->getTitle());
+
+      if ($this->isPreview) {
+        $event->setIsPreview(true);
+      } else {
+        $event
+          ->setDateCreated($xaction->getDateCreated())
+          ->setContentSource($xaction->getContentSource())
+          ->setAnchor($anchor);
+
+        $anchor++;
+      }
 
-      $anchor++;
 
       $has_deleted_comment = $xaction->getComment() &&
         $xaction->getComment()->getIsDeleted();
 
-      if ($this->getShowEditActions()) {
+      if ($this->getShowEditActions() && !$this->isPreview) {
         if ($xaction->getCommentVersion() > 1) {
           $event->setIsEdited(true);
         }
@@ -79,7 +86,7 @@ class PhabricatorApplicationTransactionView extends AphrontView {
 
         if ($xaction->hasComment() || $has_deleted_comment) {
           $has_edit_capability = PhabricatorPolicyFilter::hasCapability(
-            $viewer,
+            $user,
             $xaction,
             $can_edit);
           if ($has_edit_capability) {
@@ -134,7 +141,7 @@ class PhabricatorApplicationTransactionView extends AphrontView {
     $field = PhabricatorApplicationTransactionComment::MARKUP_FIELD_COMMENT;
 
     $engine = id(new PhabricatorMarkupEngine())
-      ->setViewer($this->viewer);
+      ->setViewer($this->getUser());
     foreach ($this->transactions as $xaction) {
       if (!$xaction->hasComment()) {
         continue;
diff --git a/src/view/layout/PhabricatorTimelineEventView.php b/src/view/layout/PhabricatorTimelineEventView.php
index 7ee899cd53..f3534d7797 100644
--- a/src/view/layout/PhabricatorTimelineEventView.php
+++ b/src/view/layout/PhabricatorTimelineEventView.php
@@ -9,11 +9,11 @@ final class PhabricatorTimelineEventView extends AphrontView {
   private $classes = array();
   private $contentSource;
   private $dateCreated;
-  private $viewer;
   private $anchor;
   private $isEditable;
   private $isEdited;
   private $transactionPHID;
+  private $isPreview;
 
   public function setTransactionPHID($transaction_phid) {
     $this->transactionPHID = $transaction_phid;
@@ -33,6 +33,14 @@ final class PhabricatorTimelineEventView extends AphrontView {
     return $this->isEdited;
   }
 
+  public function setIsPreview($is_preview) {
+    $this->isPreview = $is_preview;
+    return $this;
+  }
+
+  public function getIsPreview() {
+    return $this->isPreview;
+  }
 
   public function setIsEditable($is_editable) {
     $this->isEditable = $is_editable;
@@ -43,15 +51,6 @@ final class PhabricatorTimelineEventView extends AphrontView {
     return $this->isEditable;
   }
 
-  public function setViewer(PhabricatorUser $viewer) {
-    $this->viewer = $viewer;
-    return $this;
-  }
-
-  public function getViewer() {
-    return $this->viewer;
-  }
-
   public function setDateCreated($date_created) {
     $this->dateCreated = $date_created;
     return $this;
@@ -108,67 +107,7 @@ final class PhabricatorTimelineEventView extends AphrontView {
       $title = '';
     }
 
-    $extra = array();
-    $xaction_phid = $this->getTransactionPHID();
-
-    if ($this->getIsEdited()) {
-      $extra[] = javelin_render_tag(
-        'a',
-        array(
-          'href'  => '/transactions/history/'.$xaction_phid.'/',
-          'sigil' => 'workflow',
-        ),
-        pht('Edited'));
-    }
-
-    if ($this->getIsEditable()) {
-      $extra[] = javelin_render_tag(
-        'a',
-        array(
-          'href'  => '/transactions/edit/'.$xaction_phid.'/',
-          'sigil' => 'workflow transaction-edit',
-        ),
-        pht('Edit'));
-    }
-
-    $source = $this->getContentSource();
-    if ($source) {
-      $extra[] = id(new PhabricatorContentSourceView())
-        ->setContentSource($source)
-        ->setUser($this->getViewer())
-        ->render();
-    }
-
-    if ($this->getDateCreated()) {
-      $date = phabricator_datetime(
-        $this->getDateCreated(),
-        $this->getViewer());
-      if ($this->anchor) {
-        Javelin::initBehavior('phabricator-watch-anchor');
-
-        $anchor = id(new PhabricatorAnchorView())
-          ->setAnchorName($this->anchor)
-          ->render();
-
-        $date = $anchor.phutil_render_tag(
-            'a',
-            array(
-              'href' => '#'.$this->anchor,
-            ),
-            $date);
-      }
-      $extra[] = $date;
-    }
-
-    $extra = implode(' &middot; ', $extra);
-    if ($extra) {
-      $extra = phutil_render_tag(
-        'span',
-        array(
-          'class' => 'phabricator-timeline-extra',
-        ),
-        $extra);
-    }
+    $extra = $this->renderExtra();
 
     if ($title !== null || $extra !== null) {
       $title_classes = array();
@@ -286,4 +225,76 @@ final class PhabricatorTimelineEventView extends AphrontView {
         $content));
   }
 
+  private function renderExtra() {
+    $extra = array();
+
+    if ($this->getIsPreview()) {
+      $extra[] = pht('PREVIEW');
+    } else {
+      $xaction_phid = $this->getTransactionPHID();
+
+
+      if ($this->getIsEdited()) {
+        $extra[] = javelin_render_tag(
+          'a',
+          array(
+            'href'  => '/transactions/history/'.$xaction_phid.'/',
+            'sigil' => 'workflow',
+          ),
+          pht('Edited'));
+      }
+
+      if ($this->getIsEditable()) {
+        $extra[] = javelin_render_tag(
+          'a',
+          array(
+            'href'  => '/transactions/edit/'.$xaction_phid.'/',
+            'sigil' => 'workflow transaction-edit',
+          ),
+          pht('Edit'));
+      }
+
+      $source = $this->getContentSource();
+      if ($source) {
+        $extra[] = id(new PhabricatorContentSourceView())
+          ->setContentSource($source)
+          ->setUser($this->getUser())
+          ->render();
+      }
+
+      if ($this->getDateCreated()) {
+        $date = phabricator_datetime(
+          $this->getDateCreated(),
+          $this->getUser());
+        if ($this->anchor) {
+          Javelin::initBehavior('phabricator-watch-anchor');
+
+          $anchor = id(new PhabricatorAnchorView())
+            ->setAnchorName($this->anchor)
+            ->render();
+
+          $date = $anchor.phutil_render_tag(
+              'a',
+              array(
+                'href' => '#'.$this->anchor,
+              ),
+              $date);
+        }
+        $extra[] = $date;
+      }
+    }
+
+    $extra = implode(' &middot; ', $extra);
+    if ($extra) {
+      $extra = phutil_render_tag(
+        'span',
+        array(
+          'class' => 'phabricator-timeline-extra',
+        ),
+        $extra);
+    }
+
+    return $extra;
+  }
+
 }
diff --git a/webroot/rsrc/css/layout/phabricator-timeline-view.css b/webroot/rsrc/css/layout/phabricator-timeline-view.css
index 1521c85310..dbc3147767 100644
--- a/webroot/rsrc/css/layout/phabricator-timeline-view.css
+++ b/webroot/rsrc/css/layout/phabricator-timeline-view.css
@@ -245,3 +245,10 @@
   background: rgba(255, 255, 0, 0.50);
   box-shadow: 0 0 3px 6px rgba(255, 255, 0, 0.50);
 }
+
+.phabricator-timeline-preview-header {
+  background: #e0e3ec;
+  color: #444444;
+  padding: 4px 1.25%;
+  border: solid #c0c5d1 1px 0;
+}
diff --git a/webroot/rsrc/js/application/transactions/behavior-transaction-comment-form.js b/webroot/rsrc/js/application/transactions/behavior-transaction-comment-form.js
new file mode 100644
index 0000000000..5459584557
--- /dev/null
+++ b/webroot/rsrc/js/application/transactions/behavior-transaction-comment-form.js
@@ -0,0 +1,44 @@
+/**
+ * @provides javelin-behavior-phabricator-transaction-comment-form
+ * @requires javelin-behavior
+ *           javelin-dom
+ *           javelin-util
+ *           phabricator-shaped-request
+ */
+
+JX.behavior('phabricator-transaction-comment-form', function(config) {
+
+  var form = JX.$(config.formID);
+
+  var getdata = function() {
+    var obj = JX.DOM.convertFormToDictionary(form);
+    obj.__preview__ = 1;
+    return obj;
+  };
+
+  var onresponse = function(response) {
+    var panel = JX.$(config.panelID);
+    if (!response.xactions.length) {
+      JX.DOM.hide(panel);
+    } else {
+      JX.DOM.setContent(
+        JX.$(config.timelineID),
+        [
+          JX.$H(response.spacer),
+          JX.$H(response.xactions.join(response.spacer)),
+          JX.$H(response.spacer),
+        ]);
+      JX.DOM.show(panel);
+    }
+  };
+
+  var request = new JX.PhabricatorShapedRequest(
+    config.actionURI,
+    onresponse,
+    getdata);
+  var trigger = JX.bind(request, request.trigger);
+
+  JX.DOM.listen(form, 'keydown', null, trigger);
+
+  request.start();
+});