Theme Preview - Not working preview when front page setting

[gqevu6bsiz]

I'm sorry if there are already ticket for same problem.
Preview is not working well (not live preview/customizer).
Is this a bug? or specification?

Steps to reproduce:

1. Settings to Front page(anyting page) and select the A static page of Front page displays of Reading of Settings.
2. Access the Themes screen of Appearance with Editor(Editor already have capability to switch_themes).
3. Click to the preview button of per not activated themes.

Tested in: Google Chrome 39.0.2171.95 m

And I think this happen is affect to following.

wp-includes/canonical.php line 48 - 54

Theme preview is not have post ID. But that it become to have post ID after the Front page settings.

And I have tentatively avoiding the problem in this way.

function test_redirect_canonical( $redirect_url , $requested_url ) {
    if( get_query_var( 'preview' ) && !empty( $_GET['template'] ) && !empty( $_GET['stylesheet'] ) && !empty( $_GET['preview_iframe'] ) ) {
        if( current_user_can( 'switch_themes' ) ) {
            $redirect_url = false;
        }
    }
    return $redirect_url;
}
add_filter( 'redirect_canonical' , 'test_redirect_canonical' , 10 , 2 );

[jorbin]

This looks like to be capabilities related.

Steven has agreed to look into it.