Fix background height and padding in non-iframe editor canvas (#63222)

Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: talldan <talldanwp@git.wordpress.org>
Co-authored-by: stokesman <presstoke@git.wordpress.org>

packages/edit-post/src/components/layout/style.scss

@@ -1,4 +1,4 @@
-.edit-post-layout.has-metaboxes .edit-post-visual-editor {
+.edit-post-visual-editor:not(.is-iframed) {
 	flex: 1 0 auto;
 	height: auto;
 }