Append to body dropdown has positive z-index when inactive #3113

Overview of the problem

Buefy version: 0.9.4
Vuejs version: 2.6.12
OS/Browser: Chrome (but doesn't matter)

Description

When using a dropdown with append-to-body="true", the appended div has a positive z-index, making a part of the page unclickable as it is hidden behind this appended div.

Steps to reproduce

1. Create a dropdown
2. Add the append-to-body prop to it
3. Open your page and you will see the div has no specified z-index.

Expected behavior

The expectation would be that the div has no influence on the page while inactive.

Actual behavior

The div has an unspecified z-index, letting it capture clicks while in an inactive state.