That’s awesome news, but this is still a bug that should be fixed.
I found that simply changing:
if ( isset( $_POST['customize'] ) && isset( $_POST['theme'] ) && $_POST['customize'] == 'on' )
to:
if ( isset( $_POST['wp_customize'] ) && isset( $_POST['theme'] ) && $_POST['wp_customize'] == 'on' )
fixes the customizer issues.