index.html.erb

<h2><%= request.fullpath %></h2>

<ul>
  <% @topics.each do |topic| %>
    <li>
      <%= link_to topic.name, sandbox_path(topic_id: topic.id) %>
    </li>
  <% end %>
</ul>

<% if @topic.present? %>
<div class="tw-fixed tw-z-20 tw-top-20 tw-bg-orange-300">
<%= link_to "Close", sandbox_path %>

<h1><%= @topic.name %></h1>
<p><%= @topic.description %></p>

</div>
<% end %>

sandbox_controller.rb

# frozen_string_literal: true

class SandboxController < LoggedOutController
  before_action :load_topic, only: [:show]

  def index
    @topics = Topic.all

    if params[:topic_id]
      @topic = Topic.find(params[:topic_id])
    end
  end

  def show
  end

  def load_topic
    @topic = Topic.find(params[:id])
  end
end