[ISSUE] SelectMenu behind the markdown menu

Description

The class “SelectMenu” is presented behind the markdown menu.
Just update the attribute "z-index " of the class “.SelectMenu” from “99” to “100”

Steps to Reproduce

  1. Create a repo/branch with README.md only
  2. Open the Select menu branch list

Expected behavior: Select menu in front of the markdown header

Actual behavior: Select menu in behind of the markdown header

Versions

Windows 10, chrome, dark theme.