Sidenav: Support for headers that are not fixed and onscroll events

[bsteffl]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Maybe there would be an onscroll event that would subtract the top offset value as you scroll, and maybe an additional property that you would need to determine if you did or did not have a fixed header.

What is the current behavior?

If you set the sidenav to fixed [fixedInViewport]="true" and you offset the top value based on your header height [fixedTopGap]="64" and then you scroll down the fixed header doesn't re-position as you scroll when the header is offscreen.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-cm3u2d

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

To support a wider range of layout options for applications

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

Angular Material 5.0.0-rc3

Is there anything else we should know?

It seems like sidenav was designed to be used with a fixed header due to the overflow properties and examples provided. If this is true then maybe I'm using it in the wrong context.

[mmalerba]

currently you can accomplish this with a dynamic binding for fixedTopGap like this: https://stackblitz.com/edit/angular-cm3u2d-cij2dh?file=app/sidenav-fixed-example.ts

[simeyla]

Something exactly like this dynamic binding is done by the angular.io site when displaying an additional information bar on top. Then when you close it it animates back.

Unfortunately I'm not exactly sure how to trigger this additional bar, but I have seen it done.

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