Angular-Material Sidenav cdkScrollable

1. Add to your app module imports: ScrollDispatchModule.
2. Add cdkScrollable to your mat-sidenav-content:

<mat-sidenav-content cdkScrollable> </mat-sidenav-content>

3. In your root component:

a) inject ScrollDispatcher from @angular/cdk/overlay and subscribe to scrolling:

constructor(public scroll: ScrollDispatcher) {

    this.scrollingSubscription = this.scroll
          .scrolled()
          .subscribe((data: CdkScrollable) => {
            this.onWindowScroll(data);
          });
}

c) do something when scrolling, e.g. check the offset

private onWindowScroll(data: CdkScrollable) {
    const scrollTop = data.getElementRef().nativeElement.scrollTop || 0;
    if (this.lastOffset > scrollTop) {
      // console.log('Show toolbar');
    } else if (scrollTop < 10) {
      // console.log('Show toolbar');
    } else if (scrollTop > 100) {
      // console.log('Hide toolbar');
    }

    this.lastOffset = scrollTop;
  }

Documentation: https://material.angular.io/cdk/scrolling/api

Update Angular 9 :

Use import {ScrollingModule} from '@angular/cdk/scrolling', ScrollDispatchModule is deprecated

I opened an Issue on @angular/material some time ago and they now expose the CdkScrollable-Instance.
To use it, you need to access the MatSidenavContainer using @ViewChild(MatSidenavContainer. This instance has a public member scrollable, which is the CdkScrollable instance.
An example can be found here

Edit: As the example is not very complete and a few people are having difficulties implementing it, I'll write my own example here:

HTML:

<mat-sidenav-container>
    <mat-sidenav #sidenav>
        Sidenav Content
    </mat-sidenav>
    <div>
        Main Content
    </div>
</mat-sidenav-container>

TypeScript:

import { Component, AfterViewInit, ViewChild } from '@angular/core';
import { MatSidenavContainer } from '@angular/material';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent implements AfterViewInit  {
  @ViewChild(MatSidenavContainer) sidenavContainer: MatSidenavContainer;

    constructor() {
    }

    ngAfterViewInit() {
      console.log(this.sidenavContainer.scrollable);
    }
}

Important:

1. Don't use <mat-sidenav-content>. This tag is generated automatically and it has the cdkScrollable directive attached to it. If you use <mat-sidenav-content> in your own template, the scrollable will be undefined.
2. Use AfterViewInit instead of OnInit. As much as I know, @ViewChild is resolved in AfterViewInit, OnInit is probably too early.