Open menu should support background scrolling

[trshafer]

Bug, feature request, or proposal:

Bug? Reading the source, when having a menu open, I believe the background should be able to be scrolled using the mouse. The background can be scrolled with the arrow keys.

What is the expected behavior?

The background can scroll.

What is the current behavior?

The background cannot scroll.

What are the steps to reproduce?

1. Visit https://material.angular.io/components/menu/overview
2. Open the menu by clicking the trigger: "Menu"
3. Attempt to scroll the page (not the menu) using the mouse.

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

A dialog box should block background scrolling, but the menu should not.

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

Current

Is there anything else we should know?

It is blocked by pointer-events: auto: https://github.com/angular/material2/blob/4d82f833c7e4a22814b8e225f05ef4ba01a86ad6/src/cdk/overlay/_overlay.scss#L61.
The menu does have the scroll strategy of reposition, which I believe should mean that the background should be able to be scrolled: https://github.com/angular/material2/blob/fe37cb2551929844d5467e1b9bf45032df35981f/src/lib/menu/menu-trigger.ts#L57

[tinayuangao]

@mmalerba Is this intended behavior?

[jelbourn]

This used to be the intended behavior (the backdrop blocking scrolling), but now they should reposition

[crisbeto]

This only really happens if the scrollable element is the sidenav container, not the body. @jelbourn it is a little tricky to solve, because we'd have to disable pointer events on the backdrop which breaks the clicking outside to close an overlay. It can be resolved by disabling pointer events and using a click handler on the document to determine whether the user clicked outside, but that can be problematic too because the user could stop the event from propagating all the way up.