ci(docs-infra): reduce flakyness (#26459)

PR Close #26459

aio/src/app/app.component.spec.ts

@@ -164,10 +164,10 @@ describe('AppComponent', () => {
 
     describe('onScroll', () => {
       it('should update `tocMaxHeight` accordingly', () => {
-        expect(component.tocMaxHeight).toBeUndefined();
-
+        component.tocMaxHeight = '';
         component.onScroll();
-        expect(component.tocMaxHeight).toBeGreaterThan(0);
+
+        expect(component.tocMaxHeight).toMatch(/^\d+\.\d{2}$/);
       });
     });
 
@@ -654,12 +654,13 @@ describe('AppComponent', () => {
       it('should update the TOC container\'s `maxHeight` based on `tocMaxHeight`', () => {
         setHasFloatingToc(true);
 
-        expect(tocContainer!.style['max-height']).toBe('');
-
         component.tocMaxHeight = '100';
         fixture.detectChanges();
-
         expect(tocContainer!.style['max-height']).toBe('100px');
+
+        component.tocMaxHeight = '200';
+        fixture.detectChanges();
+        expect(tocContainer!.style['max-height']).toBe('200px');
       });
 
       it('should restrain scrolling inside the ToC container', () => {

aio/src/app/app.component.ts

@@ -349,12 +349,17 @@ export class AppComponent implements OnInit {
   @HostListener('window:scroll')
   onScroll() {
     if (!this.tocMaxHeightOffset) {
-      // Must wait until now for mat-toolbar to be measurable.
+      // Must wait until `mat-toolbar` is measurable.
       const el = this.hostElement.nativeElement as Element;
-      this.tocMaxHeightOffset =
-          el.querySelector('footer')!.clientHeight +
-          el.querySelector('.app-toolbar')!.clientHeight +
-          24; //  fudge margin
+      const headerEl = el.querySelector('.app-toolbar');
+      const footerEl = el.querySelector('footer');
+
+      if (headerEl && footerEl) {
+        this.tocMaxHeightOffset =
+            headerEl.clientHeight +
+            footerEl.clientHeight +
+            24; //  fudge margin
+      }
     }
 
     this.tocMaxHeight = (document.body.scrollHeight - window.pageYOffset - this.tocMaxHeightOffset).toFixed(2);