message: fix: prevent default dropdown click event
  Prevent default mousedown event on dropdown to keep header and footer templates working properly and do not close dropdown.
diff: @@ -16,7 +16,8 @@ import {
  ChangeDetectionStrategy,
  AfterContentInit,
  OnInit,
  - OnChanges
  + OnChanges,
  + HostListener
  } from '@angular/core';
  import { NgOption } from './ng-select.types';
  @@ -33,7 +34,7 @@ const BOTTOM_CSS_CLASS = 'ng-select-bottom';
  encapsulation: ViewEncapsulation.None,
  selector: 'ng-dropdown-panel',
  template: `
  - <div *ngIf="headerTemplate" class="ng-dropdown-header" ngProjectAs="header" header>
  + <div *ngIf="headerTemplate" class="ng-dropdown-header">
  <ng-container [ngTemplateOutlet]="headerTemplate"></ng-container>
  </div>
  <div #scroll class="ng-dropdown-panel-items scroll-host">
  @@ -42,7 +43,7 @@ const BOTTOM_CSS_CLASS = 'ng-select-bottom';
  <ng-content></ng-content>
  </div>
  </div>
  - <div *ngIf="footerTemplate" class="ng-dropdown-footer" ngProjectAs="footer" footer>
  + <div *ngIf="footerTemplate" class="ng-dropdown-footer">
  <ng-container [ngTemplateOutlet]="footerTemplate"></ng-container>
  </div>
  `,
  @@ -150,6 +151,11 @@ export class NgDropdownPanelComponent implements OnInit, OnChanges, OnDestroy, A
  el.scrollTop = d.childHeight * (d.itemsLength + 1);
  }
  + @HostListener('mousedown', ['$event'])
  + handleMousedown($event: MouseEvent) {
  + $event.preventDefault();
  + }
  +
  private _handleScroll() {
  this._disposeScrollListener = this._renderer.listen(this.scrollElementRef.nativeElement, 'scroll', () => {
  this.refresh();
  