fix(docs-infra): fix scroll position restoration error in `ScrollServ…

…ice` (#29658)

Based on Google Analytics error report, the following error happens
occasionally (15% or total errors for 2019-03):

```
Cannot read property '0' of null TypeError: at t.scrollToPosition@main.js
```

This was a result of calling [ViewportScroller#scrollToPosition()][1]
with `null`, which in turn happens when calling
[ScrollService#scrollToPosition()][2] while `this.scrollPosition` is
`null`. This can be a result of a `popstate` event without an associated
history state.

This commit fixes the error by checking whether `this.scrollPosition` is
`null`, before using it with `scrollToPosition()`.

(It also refactors away the unneeded internal `popStateFired` property.)

[1]: https://github.com/angular/angular/blob/deca6a60d/packages/common/src/viewport_scroller.ts#L101-L105
[2]: https://github.com/angular/angular/blob/deca6a60d/aio/src/app/shared/scroll.service.ts#L158-L161

PR Close #29658

aio/src/app/shared/scroll.service.spec.ts

@@ -264,16 +264,14 @@ describe('ScrollService', () => {
         location.go('/initial-url2');
         location.back();
 
-        expect(scrollService.popStateFired).toBe(true);
-        expect(scrollService.scrollPosition).toEqual([2000, 0]);
+        expect(scrollService.poppedStateScrollPosition).toEqual([2000, 0]);
         expect(scrollService.needToFixScrollPosition()).toBe(true);
       } else {
         location.go('/initial-url1');
         location.go('/initial-url2');
         location.back();
 
-        expect(scrollService.popStateFired).toBe(false); // popStateFired is always false
-        expect(scrollService.scrollPosition).toEqual([0, 0]); // scrollPosition always equals [0, 0]
+        expect(scrollService.poppedStateScrollPosition).toBe(null);
         expect(scrollService.needToFixScrollPosition()).toBe(false);
       }
 
@@ -289,21 +287,18 @@ describe('ScrollService', () => {
         location.replaceState('/initial-url1', 'hack', {scrollPosition: [2000, 0]});
 
         location.back();
-        scrollService.popStateFired = false;
-        scrollService.scrollPosition = [0, 0];
+        scrollService.poppedStateScrollPosition = [0, 0];
         location.forward();
 
-        expect(scrollService.popStateFired).toBe(true);
-        expect(scrollService.scrollPosition).toEqual([2000, 0]);
+        expect(scrollService.poppedStateScrollPosition).toEqual([2000, 0]);
         expect(scrollService.needToFixScrollPosition()).toBe(true);
       } else {
         location.go('/initial-url1');
         location.go('/initial-url2');
         location.back();
         location.forward();
 
-        expect(scrollService.popStateFired).toBe(false); // popStateFired is always false
-        expect(scrollService.scrollPosition).toEqual([0, 0]); // scrollPosition always equals [0, 0]
+        expect(scrollService.poppedStateScrollPosition).toBe(null);
         expect(scrollService.needToFixScrollPosition()).toBe(false);
       }
 

aio/src/app/shared/scroll.service.ts

@@ -19,11 +19,8 @@ export class ScrollService {
   private _topOffset: number | null;
   private _topOfPageElement: Element;
 
-  // Whether a `popstate` event has been fired (but the associated scroll position is not yet
-  // restored).
-  popStateFired = false;
   // The scroll position which has to be restored, after a `popstate` event.
-  scrollPosition: ScrollPosition = [0, 0];
+  poppedStateScrollPosition: ScrollPosition | null = null;
   // Whether the browser supports the necessary features for manual scroll restoration.
   supportManualScrollRestoration: boolean =
       !!window && ('scrollTo' in window) && ('scrollX' in window) && ('scrollY' in window) &&
@@ -72,8 +69,7 @@ export class ScrollService {
           this.removeStoredScrollPosition();
           // The `popstate` event is always triggered by a browser action such as clicking the
           // forward/back button. It can be followed by a `hashchange` event.
-          this.popStateFired = true;
-          this.scrollPosition = event.state ? event.state['scrollPosition'] : null;
+          this.poppedStateScrollPosition = event.state ? event.state.scrollPosition : null;
         }
       });
     }
@@ -160,8 +156,10 @@ export class ScrollService {
   }
 
   scrollToPosition() {
-    this.viewportScroller.scrollToPosition(this.scrollPosition);
-    this.popStateFired = false;
+    if (this.poppedStateScrollPosition) {
+      this.viewportScroller.scrollToPosition(this.poppedStateScrollPosition);
+      this.poppedStateScrollPosition = null;
+    }
   }
 
   /**
@@ -191,7 +189,7 @@ export class ScrollService {
    * Check if the scroll position need to be manually fixed after popState event
    */
   needToFixScrollPosition(): boolean {
-    return this.popStateFired && this.scrollPosition && this.supportManualScrollRestoration;
+    return this.supportManualScrollRestoration && !!this.poppedStateScrollPosition;
   }
 
   /**