Fix an issue about the algolia search overlay.

[tsanie]

• related: Muse主题下 移动端Alogila搜索有问题 iissnan/hexo-theme-next#1764

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?

Since the Muse layout specify the .site-nav is an absolute position with a z-index: 1030 in the mobile layout, that will lead its child .algolia-popup would be covered by the .algolia-pop-overlay (which with a z-index: 2080)

Issue Number(s): iissnan/hexo-theme-next#1764

What is the new behavior?

Set the .algolia-pop-overlay to be hidden in the mobile layout, because the .algolia-popup will fill the entire screen.

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

How to use?

N/A

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

Good fix!

P.S. Check it in roadmap: #67.