This is a simple plugin that will make the Global Fade option persistent through Combat, Targeting, Setting Focus, and Loss of Health. Essentially, that means the other events that cause your action bars to suddenly show up will no longer be triggered.

This plugin requires that you use Global Fade options in ElvUI.
Your action bars will only be displayed on mouseover regardless of status/event.

The only exception to this is when a player is in a Vehicle. This plugin will automatically display the Vehicle action bar (no mouseover needed).

Steps for Install/Configuration:

Install ElvUI (https://www.tukui.org/download.php?ui=elvui)
Install this plugin.
Set the Global Fade Transparency to 100% in the ActionBars > General Options pane
Check "Inherit Global Fade" under the ActionBars > Bar 1 pane (and check that for any other bar you want to fade out with mouse over).
Make sure you have unchecked "Mouse Over" for each bar you want this to be applied to as well.

Screenshot with mouseover:

Screenshot in combat, notice the bars are still faded away:
In-Combat

ElvUI_GlobalFadePersist