Twenty Nineteen: Add margins to editor iframe content.

At screen widths of at least 768px (tablet size), Twenty Nineteen should have a `max-width` and side margins for content in the post editor.

This commit applies that style to the new `.block-editor-iframe__body` class, resolving an issue where full-width blocks had a horizontal scrollbar in the editor iframe.

Props sabernhardt, smit08, nidhidhandhukiya, mukesh27, darshitrajyaguru97, oglekler, poena, shailu25, nicolefurlan.
Fixes #59449.
Built from https://develop.svn.wordpress.org/trunk@56832


git-svn-id: http://core.svn.wordpress.org/trunk@56344 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentynineteen/style-editor.css

@@ -605,7 +605,7 @@ body .wp-block.aligncenter {
 }
 
 @media only screen and (min-width: 768px) {
-  body.block-editor-writing-flow,
+  body.block-editor-iframe__body, body.block-editor-writing-flow,
   body .block-editor-writing-flow {
     max-width: 80%;
     margin: 0 10%;

wp-content/themes/twentynineteen/style-editor.scss

@@ -36,6 +36,7 @@ body {
 
 	@include media(tablet) {
 
+		&.block-editor-iframe__body,
 		&.block-editor-writing-flow,
 		.block-editor-writing-flow {
 			max-width: 80%;

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-beta3-56831';
+$wp_version = '6.4-beta3-56832';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.