Bug: tagName Lowercasing in @shikijs/twoslash renderer for custom popup

[Zih0]

Problem Description

When using the @shikijs/twoslash transformer, I encountered an issue where the tagName provided in the renderer configuration is automatically converted to lowercase.

Example

Here is the configuration where the issue occurs:

transformers: [
  transformerTwoslash({
    explicitTrigger: true,
    renderer: rendererRich({
      hast: {
        hoverPopup: {
          tagName: 'Popup'
        },
        // additional configuration
      }
    }),
  }),
],

In this case, the expected behavior is that a Popup node is generated, which would correspond to the Popup component in MDX. However, the node is being converted to lowercase (popup).

I think that during the process where the HTML generated by highlighter.codeToHtml is converted back to hast, the tagName is being transformed to lowercase.

You can reproduce this issue by checking the following example:
StackBlitz Example

twoslash custom popup usage:
https://github.com/shikijs/shiki/blob/main/packages/vitepress-twoslash/src/renderer-floating-vue.ts
https://github.com/fuma-nama/fumadocs/blob/38e2b602d6c8e818cae8d01037c7c2245be93803/packages/twoslash/src/index.ts