Fix scroll in sidebar

[ljcbaby]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Issue resolved: #307

What is the new behavior?

• Link to demo site with this changes:
• Screenshots with this changes:

How to use?

In NexT _config.yml:

[coveralls]

Coverage remained the same at 94.468% when pulling 8d20da8 on scroll-in-sidebar into ec312ec on master.

[ljcbaby]

@tusooa Can you help me to test this in " Muse | Mist " ?

[welcome]

Congrats on merging your first pull request here! 🎉 How awesome!