MC-17814: Insert Widget Slide Out

Author: danmooney2

app/code/Magento/Backend/view/adminhtml/templates/widget/form/renderer/fieldset.phtml

@@ -53,6 +53,9 @@ if ($isField) {
 
     <?php if (!$element->getNoContainer()) : ?>
         <fieldset class="<?=  $block->escapeHtmlAttr($cssClass) ?>" id="<?=  $block->escapeHtmlAttr($id) ?>">
+        <?php if (strlen($element->getBeforeElementHtml())) : ?>
+            <?= $element->getBeforeElementHtml() ?>
+        <?php endif ?>
         <?php if ($element->getLegend() && !$isWrapped) : ?>
             <legend class="<?= /* @noEscape */ $isField ? 'label admin__field-label' : 'admin__legend legend' ?>">
                 <span><?= $block->escapeHtml($element->getLegend()) ?></span>

app/code/Magento/Widget/Block/Adminhtml/Widget/Form.php

@@ -53,6 +53,7 @@ protected function _prepareForm()
         $fieldset = $form->addFieldset(
             'base_fieldset',
             [
+                'before_element_html' => '<div data-role="messages"></div>',
                 'comment' => __('Inserting a widget does not create a widget instance.'),
             ]
         );