CMS editor: opening an .mdx file gets an error

Inspect components/cms/FileEditor.tsx around lines 335-359, where the open-file tabs are rendered, and reproduce the issue by opening an .mdx file in the CMS at app/admin/(dashboard)/cms/page.tsx. The fix is complete when the file tab controls no longer produce a nested-button hydration error in the console.

## Error Type
Console Error

## Error Message
In HTML, <button> cannot be a descendant of <button>.
This will cause a hydration error.

  ...
    <RedirectBoundary>
      <RedirectErrorBoundary router={{...}}>
        <InnerLayoutRouter url="/admin/cms" tree={[...]} params={{}} cacheNode={{rsc:<Fragment>, ...}} ...>
          <SegmentViewNode type="page" pagePath="/toolkata/...">
            <SegmentTrieNode>
            <ClientPageRoot Component={function CMSPage} serverProvidedParams={{...}}>
              <CMSPage params={Promise} searchParams={Promise}>
                <div className="flex flex-...">
                  <div>
                  <div className="flex-1 fle...">
                    <div>
                    <div className="flex-1 fle...">
                      <FileEditor files={[...]} activeFileIndex={0} ...>
                        <div className="flex flex-...">
                          <div className="flex items..." role="tablist" aria-label="Open files">
>                           <button
>                             type="button"
>                             role="tab"
>                             aria-selected={true}
>                             aria-controls="editor-panel-0"
>                             onClick={function onClick}
>                             onMouseDown={function onMouseDown}
>                             className="group flex items-center gap-1.5 px-3 py-1.5 text-xs font-mono rounded-t trans..."
>                           >
                              <span>
                              <span>
>                             <button
>                               type="button"
>                               onClick={function onClick}
>                               className="ml-1 p-0.5 rounded text-[var(--color-text-dim)] hover:text-[var(--color-err..."
>                               aria-label="Close glossary.ts"
>                             >
                          ...
                    ...
                  ...
          ...
        ...



    at button (<anonymous>:null:null)
    at <unknown> (components/cms/FileEditor.tsx:359:13)
    at Array.map (<anonymous>:null:null)
    at FileEditor (components/cms/FileEditor.tsx:335:16)
    at CMSPage (app/admin/(dashboard)/cms/page.tsx:740:13)

## Code Frame
  357 |               />
  358 |             )}
> 359 |             <button
      |             ^
  360 |               type="button"
  361 |               onClick={(e) => {
  362 |                 e.stopPropagation()

Next.js version: 16.1.4 (Turbopack)