[overlay] 0.48 opacity of showing backdrop is unexpected

[willshowell]

Bug, feature request, or proposal:

Proposal

Expected behavior

Providing a custom backdrop class with background-color: red would result in a fully opaque backdrop.

Current behavior

Be default, that backdrop will be at 0.48 opacity. To make it fully opaque,

1. You must explicitly set opacity: 1
2. You must do so appropriately within .cdk-overlay-backdrop-showing which requires at least a little digging through the DOM or source

https://stackblitz.com/edit/angular-material2-issue-t9xu95?file=styles.scss

Relevant code

https://github.com/angular/material2/blob/26bbeb2cda3ab9d03c00dbb45e2317a48feed5d8/src/cdk/overlay/_overlay.scss#L8-L9

https://github.com/angular/material2/blob/26bbeb2cda3ab9d03c00dbb45e2317a48feed5d8/src/cdk/overlay/_overlay.scss#L64-L68

Motivation

Showing a backdrop which is darker than 0.48 opacity it slightly more troublesome than it could be.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Latest

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