[button ]v9.0.1 breaks layout with custom CSS `gap`

[ZebraFlesh]

Steps to reproduce

Steps:

1. Create a Button with a startIcon and a gap: 20px style
2. Upgrade to 9.0.1
3. Observe that layout has been broken

Example:

<Button startIcon={<AddBoxIcon />}
  sx={{
     border: '1px solid red',
     gap: '20px',
   }}
>
  My Button
</Button>

Current behavior

9.0.1: https://stackblitz.com/edit/github-5wywe1mb-abkffyvk?file=src%2FApp.tsx

Expected behavior

9.0.0: https://stackblitz.com/edit/github-5wywe1mb-o1rdasp8?file=src%2FApp.tsx

Context

PR #48332 fixes a bug in the Button component by introducing a ::before pseudo element that supplies a content: '\200b' attribute (non-breaking space). This additional content breaks existing layouts. I expect a patch version upgrade to not break my layouts.

This happens across latest Chrome, Chrome Canary, Safari, and Firefox.

Your environment

npx @mui/envinfo

 System:
    OS: Linux 5.0 undefined
  Binaries:
    Node: 22.22.0 - /usr/local/bin/node
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  Browsers:
    Chrome: Not Found
    Firefox: Not Found
  npmPackages:
    @emotion/react: latest => 11.14.0 
    @emotion/styled: latest => 11.14.1 
    @mui/core-downloads-tracker:  9.0.1 
    @mui/icons-material: 9.0.1 => 9.0.1 
    @mui/material: 9.0.1 => 9.0.1 
    @mui/private-theming:  9.0.1 
    @mui/styled-engine:  9.0.0 
    @mui/system:  9.0.1 
    @mui/types:  9.0.0 
    @mui/utils:  9.0.1 
    @types/react: latest => 19.2.14 
    react: latest => 19.2.6 
    react-dom: latest => 19.2.6 
    typescript: latest => 6.0.3 

Search keywords: 9.0.1 button

[mj12albert]

Workaround for now:

<Button
  startIcon={<AddBoxIcon />}
  sx={{
    gap: '20px',
    '&::before': { content: 'none' },
  }}
>
  My Button
</Button>

[github-actions]

This issue has been closed. If you have a similar problem but not exactly the same, please open a new issue.
Now, if you have additional information related to this issue or things that could help future readers, feel free to leave a comment.

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