Open Menu positioning breaks when scrolling below top of root trigger

[rickysandoval]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

It's expected that the menu stays positioned with the trigger

What is the current behavior?

When scrolling below the top of the trigger, the position of the open menu jumps down in proportion to the space between the trigger and top of the page + additional scroll

What are the steps to reproduce?

1. Open menu
2. Scroll down the page

https://stackblitz.com/edit/angular-rban4u

What is the use-case or motivation for changing an existing behavior?

user experience

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

Angular 6.0.3
Angular Material 6.2.1, CDK 6.2.1
Typescript 2.7.2
OSX High Sierra
Chrome, Safari, Firefox

Is there anything else we should know?

[crisbeto]

Closing as a duplicate of #11365.

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