MC-17948: Newsletter template preview show small section with scroll when image added

Author: Oleksandr Dubovyk

app/code/Magento/Newsletter/view/adminhtml/templates/preview/iframeswitcher.phtml

@@ -17,6 +17,7 @@
     <iframe
             name="preview_iframe"
             id="preview_iframe"
+            class="preview_iframe"
             frameborder="0"
             title="<?= $block->escapeHtmlAttr(__('Preview')) ?>"
             width="100%"