In certain situations (migrations, etc.) the admin login page style urls are incorrect

[mongoose643]

I found this bug after launching a wordpress site that was initially set up in a subdirectory on a different domain and then moved to the web root of the primary domain. I don't know for sure that this procedure is what brought out the bug.

On a more technical note, the bug is that the WP_Styles::_css_href() method has an if statement condition that checks the SRC URI for the stylesheet. Unlike other places in the system, this conditional check did not account for root relative URIs by checking to see if the first character was a forward slash "/".

Here is the modified version of this method:

function _css_href( $src, $ver, $handle ) {
    if ( !preg_match('|^https?://|', $src) && substr($src, 0, 1) != '/' && ! ( $this->content_url && 0 === strpos($src, $this->content_url) ) ) {
        $src = $this->base_url . $src;
    }

    $src = add_query_arg('ver', $ver, $src);
    $src = apply_filters( 'style_loader_src', $src, $handle );
    return esc_url( $src );
}

We need more details of the configuration here. You likely you have your home URL set to '/', which isn't supported. (It should be a FQ URL.)

Closing as invalid. Re-open with more feedback.