modify the bug of sidebar-inner

[iNiL0119]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

init the blog and link to localhost:4000 with Next, click the title Hello World then show the detail of the article, then scorll to bottom and refresh(MacOS commond + shift +R / commond + R) repeatedly, and you can see the class element 'sidebar-inner' has class name 'affix-bottom' with top 915px sometimes.
Issue Number(s): N/A

What is the new behavior?

Description about this pull, in several words...

• Screens with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

...

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

NexT is rebased into organization repo.
If you want new feature, please, create pull in NexT v6.x repo.
There is instructions on English or Chinese how to update from v5.1.x to v 6.x

You also may read this for details.