[BUG] Split View doesn't work with tabMods.uc.js

[Aridow]

Link to the file containing the bug
https://github.com/aminomancer/uc.css.js/blob/master/resources/script-override/tabMods.uc.js

Describe the bug
With tabMods.uc.js, Split View doesn't work. When trying to close the tabs, there's an empty space left. Blue outline appear randomly on every tabs.

To Reproduce
Steps to reproduce the behavior:

1. Install tabMods.uc.js
2. Right click on a tab
3. Click on "Add Split View"

Expected behavior
Two tabs should appear at the same time.

Screenshots
If applicable, add screenshots to help explain your problem.
After clicking on "Add Split View":

After closing Youtube:

Even after closing the Split View, the blue outline randomly remains on other tabs:

Desktop (please complete the following information):

• OS: Windows 11
• Firefox update channel: Nightly
• Version: 151.0a1
• Build ID: 20260330213624

Additional context
Tested in a new profile, with all CSS/scripts disabled except tabMods.uc.js.
Maybe I should specify that my chrome.manifest contains the following line, as written in tabMods.uc.js:
override chrome://browser/content/tabbrowser/tab.js ../resources/script-override/tabMods.uc.js
instead of the one written in README.md, which doesn't seem to have any effect:
override chrome://browser/content/tabbrowser-tab.js ../resources/tabMods.uc.js