fix(overlay): transparent overlay not blocking scroll on Firefox 57 (#9446)
fix(overlay): transparent overlay not blocking scroll on Firefox 57 (#9446)

As of Firefox 57, it looks like elements with a transparent background won't capture scroll events. These changes work around the issue by not setting the background color, but using `opacity` to hide the element while keeping it scrollable.

Fixes #8924.
Eddman/material2,tinayuangao/material2,josephperrott/material2,jelbourn/material2,angular/components,DevVersion/material2,jelbourn/material2,emoralesb05/material2,DevVersion/material2,amcdnl/material2,Eddman/material2,crisbeto/material2,josephperrott/material2,fairfaxmedia/material2,Jefiozie/material2,andrewseguin/material2,emoralesb05/material2,fairfaxmedia/material2,angular/components,amcdnl/material2,emoralesb05/material2,Jefiozie/material2,josephperrott/material2,Jefiozie/material2,crisbeto/material2,Jefiozie/material2,ppham27/material2,tinayuangao/material2,fairfaxmedia/material2,josephperrott/material2,Jefiozie/material2,mmalerba/material2,emoralesb05/material2,Eddman/material2,josephperrott/material2,andrewseguin/material2,andrewseguin/material2,Eddman/material2,mmalerba/material2,jelbourn/material2,ppham27/material2,fairfaxmedia/material2,jelbourn/material2,mmalerba/material2,mmalerba/material2,andrewseguin/material2,mmalerba/material2,crisbeto/material2,Eddman/material2,tinayuangao/material2,ppham27/material2,ppham27/material2,andrewseguin/material2,emoralesb05/material2,tinayuangao/material2,DevVersion/material2,tinayuangao/material2,amcdnl/material2,angular/components,DevVersion/material2,DevVersion/material2,amcdnl/material2,ppham27/material2,crisbeto/material2,jelbourn/material2,angular/components,fairfaxmedia/material2,amcdnl/material2