refactor(cdk/scrolling): expose rendered content offset stream

Exposes the `renderedContentOffset` stream on the virtual scroll viewport since it can be useful for users outside of the CDK table.

Author: crisbeto

goldens/cdk/scrolling/index.api.md

@@ -199,6 +199,7 @@ export class CdkVirtualScrollViewport extends CdkVirtualScrollable implements On
     ngOnInit(): void;
     get orientation(): "horizontal" | "vertical";
     set orientation(orientation: 'horizontal' | 'vertical');
+    readonly renderedContentOffset: Observable<number>;
     readonly renderedRangeStream: Observable<ListRange>;
     // (undocumented)
     scrollable: CdkVirtualScrollable;

src/cdk/scrolling/virtual-scroll-viewport.ts

@@ -35,7 +35,6 @@ import {
   asapScheduler,
   Observable,
   Observer,
-  OperatorFunction,
   Subject,
   Subscription,
 } from 'rxjs';
@@ -103,15 +102,7 @@ export class CdkVirtualScrollViewport extends CdkVirtualScrollable implements On
 
   /** Emits when the rendered range changes. */
   private readonly _renderedRangeSubject = new Subject<ListRange>();
-
-  /**
-   * Emits the offset from the start of the viewport to the start of the rendered data (in pixels).
-   */
-  private readonly _renderedContentOffsetRenderedSubject = new Subject<number | null>();
-  readonly _renderedContentOffsetRendered = this._renderedContentOffsetRenderedSubject.pipe(
-    filter(offset => offset !== null) as OperatorFunction<number | null, number>,
-    distinctUntilChanged(),
-  );
+  private readonly _renderedContentOffsetSubject = new Subject<number | null>();
 
   /** The direction the viewport scrolls. */
   @Input()
@@ -151,6 +142,14 @@ export class CdkVirtualScrollViewport extends CdkVirtualScrollable implements On
   /** A stream that emits whenever the rendered range changes. */
   readonly renderedRangeStream: Observable<ListRange> = this._renderedRangeSubject;
 
+  /**
+   * Emits the offset from the start of the viewport to the start of the rendered data (in pixels).
+   */
+  readonly renderedContentOffset: Observable<number> = this._renderedContentOffsetSubject.pipe(
+    filter(offset => offset !== null),
+    distinctUntilChanged(),
+  );
+
   /**
    * The total size of all content (in pixels), including content that is not currently rendered.
    */
@@ -547,10 +546,7 @@ export class CdkVirtualScrollViewport extends CdkVirtualScrollable implements On
       // string literals, a variable that can only be 'X' or 'Y', and user input that is run through
       // the `Number` function first to coerce it to a numeric value.
       this._contentWrapper.nativeElement.style.transform = this._renderedContentTransform;
-
-      // Emit the offset to rendered content start when it is in sync with what is rendered in the
-      // DOM.
-      this._renderedContentOffsetRenderedSubject.next(this.getOffsetToRenderedContentStart());
+      this._renderedContentOffsetSubject.next(this.getOffsetToRenderedContentStart());
 
       afterNextRender(
         () => {