zIndex bug in `SiteHeader, FixedToolbar`

[jijiseong]

Description

There is zIndex bug between SiteHeader, FixedToolbar and FloatingToolbar

Steps to Reproduce

1. Go to https://platejs.org/

2. Make the floating toolbar float.
   

3. Scroll down
   

The floating-toolbar overlaps the SiteHeader and FixedToolbar.

Sandbox

Expected Behavior

The floating toolbar should be positioned under the header.

Environment

• slate:
• slate-react:
• browser: chrome

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