Centered bookmar bar doesn't compensate for hidden overflow

### Describe the bug

If there's any bookmarks that are hidden due to the not fitting on the bookmark bar, it goes off centre with the `userChrome.centered.bookmarkbar` option enabled.

### Expected behavior

It should always be centred.

### Screenshots

https://user-images.githubusercontent.com/65787561/216786324-dde18aff-5cd7-4d5a-aa3f-4271042ee2ff.mp4

### OS

Windows 10

### OS - Others

_No response_

### Firefox Version

109.0-1

### Distribution

- [x] [Original Lepton](https://github.com/black7375/Firefox-UI-Fix)
- [ ] [Lepton's photon style](https://github.com/black7375/Firefox-UI-Fix/tree/photon-style)
- [ ] [Lepton's proton style](https://github.com/black7375/Firefox-UI-Fix/tree/proton-style)

### Theme

- [ ] Light
- [X] Dark
- [ ] Alpenglow (System: Light)
- [ ] Alpenglow (System: Dark)
- [ ] System Default
- [ ] Others: (e.g. [Photon Colors](https://addons.mozilla.org/en-US/firefox/addon/photon-colors/), [Google Chrome Dark](https://addons.mozilla.org/en-US/firefox/addon/google-chrome-dark/), [Fairytale Of Nature](https://addons.mozilla.org/en-US/firefox/addon/fairytale-of-nature/))

### Theme - More Info

_No response_

### `user.js` setup

User options

userChrome.centered.bookmarkbar: true

### Additional context

_No response_

Reproduce the issue in Firefox 109 on Windows 10 with `userChrome.centered.bookmarkbar: true` and enough bookmarks to overflow the bar. Investigate the SCSS responsible for the centered bookmark bar and verify that the visible bookmarks remain centered when some are hidden.

Written by the indexing model from the issue text.