use trackBy to prevent all years from being rebuilt

src/app/degree-planner/degree-planner.component.html

@@ -157,9 +157,10 @@
           style="margin: 24px">
           <div id="year-mask" *ngIf="(isLoadingPlan$ | async)"></div>
           <mat-accordion multi="true">
-            <ng-container *ngFor="let yearCode of (yearCodes$ | async)">
-              <cse-year-container [yearCode]="yearCode"></cse-year-container>
-            </ng-container>
+            <cse-year-container
+              *ngFor="let yearCode of yearCodes$ | async; trackBy: trackYearCodes"
+              [yearCode]="yearCode">
+            </cse-year-container>
           </mat-accordion>
         </div>
       </mat-sidenav-content>

src/app/degree-planner/degree-planner.component.ts

@@ -235,6 +235,10 @@ export class DegreePlannerComponent implements OnInit, OnDestroy {
   public closeCourseSearch() {
     this.store.dispatch(new CloseCourseSearch());
   }
+
+  public trackYearCodes(_index: number, yearCode: YearCode) {
+    return yearCode.toString();
+  }
 }
 
 const isntUndefined = <T>(anything: T | undefined): anything is T => {