Thick Extra Border from top when using `bookmarkbar.multi_row` with some other settings.

### Describe the bug

I am using hide-tabbar , autohide - navbar, autohide bookmarkbar. A thick bar remains on top. Whenever the multi-row value crosses 1 , this happens.

### Expected behavior

AutoHIde working perfectly.

### Screenshots


### OS

Linux

### OS - Others

_No response_

### Firefox Version

v119.0 (64-bit)

### 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.js setup

```javascript
user_pref("userChrome.autohide.tabbar", false);
user_pref("userChrome.autohide.navbar", true);
user_pref("userChrome.autohide.bookmarkbar", true);
user_pref("userChrome.autohide.toolbar_overlap", true);
user_pref("userChrome.autohide.toolbar_overlap.allow_layout_shift", false);
user_pref("userChrome.autohide.page_action", true);

user_pref("userChrome.hidden.tabbar", true);
user_pref("userChrome.hidden.sidebar_header", true);
user_pref("userChrome.hidden.sidebar_header.vertical_tab_only", false);
user_pref("userChrome.hidden.urlbar_iconbox", true);
user_pref("userChrome.hidden.bookmarkbar_label", false);
user_pref("userChrome.hidden.disabled_menu", true);

user_pref("userChrome.centered.urlbar", true);
user_pref("userChrome.centered.bookmarkbar", true);

user_pref("userChrome.urlView.always_show_page_actions", true);

user_pref("userChrome.sidebar.overlap", false);

user_pref("userChrome.bookmarkbar.multi_row", true);
```

### Additional context

_No response_

Reproduce the issue on Linux with Firefox 119 using the `user.js` settings shown, especially `userChrome.bookmarkbar.multi_row` and the autohide options. Inspect the Firefox UI styling related to the multi-row bookmark bar and autohide behavior; done means the extra top border is absent while autohide and multi-row behavior still work.

Written by the indexing model from the issue text.