Add a hidden submit button to the Appearance -> Header screen so keyboard users can submit the form correctly when a default image provided by the theme is in use.

Fixes #20880.
Props kovshenin, joedolson.


git-svn-id: https://develop.svn.wordpress.org/trunk@30095 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-header.php

@@ -615,6 +615,7 @@ class Custom_Image_Header {
 </table>
 
 <form method="post" action="<?php echo esc_url( add_query_arg( 'step', 1 ) ) ?>">
+<?php submit_button( null, 'screen-reader-text', 'save-header-options', false ); ?>
 <table class="form-table">
 <tbody>
 	<?php if ( get_uploaded_header_images() ) : ?>