Fix blur behind closed registration message (#12442)

* Fix closed registration message blur

* Adjust overlay margins to account for blur

Fixes #12425

app/javascript/styles/mastodon/forms.scss

@@ -513,6 +513,10 @@ code {
   &__overlay-area {
     position: relative;
 
+    &__blurred form {
+      filter: blur(2px);
+    }
+
     &__overlay {
       position: absolute;
       top: 0;
@@ -523,8 +527,10 @@ code {
       justify-content: center;
       align-items: center;
       background: rgba($ui-base-color, 0.65);
-      backdrop-filter: blur(2px);
       border-radius: 4px;
+      margin-left: -4px;
+      margin-top: -4px;
+      padding: 4px;
 
       &__content {
         text-align: center;

app/views/about/_registration.html.haml

@@ -1,5 +1,5 @@
+.simple_form__overlay-area{ class: (closed_registrations? && @instance_presenter.closed_registrations_message.present?) ? 'simple_form__overlay-area__blurred' : '' }
   = simple_form_for(new_user, url: user_registration_path, namespace: 'registration') do |f|
-  .simple_form__overlay-area
     %p.lead= t('about.federation_hint_html', instance: content_tag(:strong, site_hostname))
 
     .fields-group