Add confirmation when redirecting logged-out requests to permalink

app/controllers/concerns/web_app_controller_concern.rb

@@ -21,10 +21,10 @@ module WebAppControllerConcern
   def redirect_unauthenticated_to_permalinks!
     return if user_signed_in? && current_account.moved_to_account_id.nil?
 
-    redirect_path = PermalinkRedirector.new(request.path).redirect_path
-    return if redirect_path.blank?
+    @redirect_path = PermalinkRedirector.new(request.path).redirect_path
+    return if @redirect_path.blank?
 
     expires_in(15.seconds, public: true, stale_while_revalidate: 30.seconds, stale_if_error: 1.day) unless user_signed_in?
-    redirect_to(redirect_path)
+    render 'redirects/show', layout: 'application'
   end
 end

app/javascript/styles/mastodon/containers.scss

@@ -104,3 +104,59 @@
     margin-inline-start: 10px;
   }
 }
+
+.redirect {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  height: 100vh;
+  font-size: 14px;
+  line-height: 18px;
+
+  &__logo {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    margin-bottom: 30px;
+
+    img {
+      height: 48px;
+    }
+  }
+
+  &__message {
+    text-align: center;
+
+    h1 {
+      font-size: 17px;
+      line-height: 22px;
+      font-weight: 700;
+      margin-bottom: 30px;
+    }
+
+    p {
+      margin-bottom: 30px;
+
+      &:last-child {
+        margin-bottom: 0;
+      }
+    }
+
+    a {
+      color: $highlight-text-color;
+      font-weight: 500;
+      text-decoration: none;
+
+      &:hover,
+      &:focus,
+      &:active {
+        text-decoration: underline;
+      }
+    }
+  }
+
+  &__link {
+    margin-top: 15px;
+  }
+}

app/views/redirects/show.html.haml

@@ -0,0 +1,8 @@
+.redirect
+  .redirect__logo
+    = link_to render_logo, root_path
+
+  .redirect__message
+    %h1= t('redirects.title', instance: site_hostname)
+    %p= t('redirects.prompt')
+    %p= link_to @redirect_path, @redirect_path, rel: 'noreferrer noopener'

config/locales/en.yml

@@ -1546,6 +1546,9 @@ en:
     errors:
       limit_reached: Limit of different reactions reached
       unrecognized_emoji: is not a recognized emoji
+  redirects:
+    prompt: If you trust this link, click it to continue.
+    title: You are leaving %{instance}.
   relationships:
     activity: Account activity
     confirm_follow_selected_followers: Are you sure you want to follow selected followers?