Plugin: Filter safe CSS to allow column flex-basis CSS attribute

lib/compat.php

@@ -0,0 +1,26 @@
+<?php
+/**
+ * Temporary compatibility shims for features present in Gutenberg, pending
+ * upstream commit to the WordPress core source repository. Functions here
+ * exist only as long as necessary for corresponding WordPress support, and
+ * each should be associated with a Trac ticket.
+ *
+ * @package gutenberg
+ */
+
+/**
+ * Filters allowed CSS attributes to include `flex-basis`, included in saved
+ * markup of the Column block.
+ *
+ * @since 5.7.0
+ *
+ * @param string[] $attr Array of allowed CSS attributes.
+ *
+ * @return string[] Filtered array of allowed CSS attributes.
+ */
+function gutenberg_safe_style_css_column_flex_basis( $attr ) {
+	$attr[] = 'flex-basis';
+
+	return $attr;
+}
+add_filter( 'safe_style_css', 'gutenberg_safe_style_css_column_flex_basis' );

lib/load.php

@@ -24,6 +24,7 @@
 	require dirname( __FILE__ ) . '/rest-api.php';
 }
 
+require dirname( __FILE__ ) . '/compat.php';
 require dirname( __FILE__ ) . '/blocks.php';
 require dirname( __FILE__ ) . '/client-assets.php';
 require dirname( __FILE__ ) . '/demo.php';