Issue Summary

When Confluence Data Center transforms legacy code block markup inside HTML <details> elements, the new code block UI is rendered but the code content is dropped. This occurs with content produced by the Markdown for Confluence app (and likely any app/output that emits legacy code macro markup) when a fenced code block appears inside <details>.

Outside <details>, the same code block renders correctly with language and syntax highlighting.

Steps to Reproduce

1. Install and enable the Markdown for Confluence app.
2. Create a new Confluence page.
3. Insert the Markdown macro (e.g. using /markdown).
4. In the macro body, add the following Markdown content and enable the macro parameter allowHtml:

   <details>
       <summary style="display: list-item;"><b>My Summary 1</b></summary>
       Summary one details
   </details>
   <details>
       <summary style="display: list-item;"><b>My Summary 2</b></summary>
       Summary 2 details
   </details>
   <details>
       <summary style="display: list-item;"><b>My Summary 3 - with code block inside</b></summary>
   ```ruby
   require 'redcarpet'
   markdown = Redcarpet.new("Hello World!")
   puts markdown.to_html
   ```
   </details>
   ```ruby 
   require 'redcarpet' 
   markdown = Redcarpet.new("Hello World!") 
   puts markdown.to_html
   ```

1. Save the macro configuration and publish the page.
2. Expand “My Summary 3 - with code block inside” and compare the code block inside <details> with the code block rendered outside <details>.

(Optional minimal platform-only repro, without the app: )
Use the HTML macro to embed the equivalent HTML that the app produces, including a <pre class="syntaxhighlighter-pre" ...> block inside and outside <details>, and observe the same behavior.

Expected Results

1. The code block inside the <details> section should render identically to the one outside <details>:
2. The code content is visible and intact.
3. The language is preserved (e.g. language-ruby).
4. Syntax highlighting tokens are generated.
5. The new code block UI (wrap / copy buttons) works as expected.

Actual Results

• The code block inside <details> is rendered as an empty block:
• Confluence transforms the legacy markup into the new DS/Prism code block structure (buttons, container, etc.), but:
• The <code> element is rendered as:

<code class="language-text" style="white-space: pre;">
  <span class=""></span>
</code>

• No code content is present.
  *. No syntax highlighting tokens are present.
• The code block outside <details> renders correctly with:

class="language-ruby"

Full code content and Prism tokens.

This indicates that the internal transformation that upgrades legacy

<pre class="syntaxhighlighter-pre" ...>

blocks to the new code block UI is executed, but fails to carry over the code content when the block is nested inside <details>.

Workaround

For content that may be nested inside HTML <details>:
Avoid emitting the legacy Code Block macro markup (e.g. <pre class="syntaxhighlighter-pre" data-syntaxhighlighter-params="brush: ruby; ...">).
Instead, render code blocks inside <details> as plain HTML <pre><code> elements so they are not picked up by the broken transformation:

<details>
  <summary style="display: list-item;"><b>My Summary 3 - with code block inside</b></summary>
  <pre><code class="language-ruby">
require 'redcarpet'
markdown = Redcarpet.new("Hello World!")
puts markdown.to_html
  </code></pre>
</details

• The code content will display correctly (without the new DS code block toolbar and syntax highlighting, but fully visible and copyable).
• If this workaround is not applicable, users should avoid combining <details> with legacy-style code block markup until the transformation logic is fixed.