fix losing focusing when pressing Page Down

Author: zarend

src/material/datepicker/month-view.ts

@@ -342,6 +342,7 @@ export class MatMonthView<D> implements AfterContentInit, OnChanges, OnDestroy {
       this.activeDateChange.emit(this.activeDate);
     }
 
+    this._focusActiveCell();
     // Prevent unexpected default actions such as form submission.
     event.preventDefault();
   }

src/material/datepicker/multi-year-view.ts

@@ -293,6 +293,7 @@ export class MatMultiYearView<D> implements AfterContentInit, OnDestroy {
       this.activeDateChange.emit(this.activeDate);
     }
 
+    this._focusActiveCell();
     // Prevent unexpected default actions such as form submission.
     event.preventDefault();
   }

src/material/datepicker/year-view.ts

@@ -280,6 +280,7 @@ export class MatYearView<D> implements AfterContentInit, OnDestroy {
       this.activeDateChange.emit(this.activeDate);
     }
 
+    this._focusActiveCell();
     // Prevent unexpected default actions such as form submission.
     event.preventDefault();
   }