Description

Setting custom slugify function "uslugify" from pymodown breaks header anchors block at the right.

markdown_extensions:
    ...
    - toc:
          permalink: '#'
          slugify: !!python/name:pymdownx.slugs.uslugify
    ...

Expected behavior

When scrolling down headers are fading out. This gives a visual understanding of current position on the page.

Actual behavior

When scrolling, the browser console spams the following error:

Steps to reproduce the bug

1. In mkdocs.yml set a custom slugify function ("uslugify" in my case)
2. Add some cyrillic headers in index.md
3. Anchor links are not fading and the console spams the error when scrolling the page