CSS: Position loading indicator in the center of t

Question

How can I position my loading indicator in the center of the screen. Currently I'm using a little placeholder and it seems to work fine. However, when I scroll down, the loading indicator stays right in that predefined position. How can I make it follow the scrolling so that it always sits on top??

#busy
{
    position: absolute;
    left: 50%;
    top: 35%;
    display: none;
    background: transparent url("../images/loading-big.gif");
    z-index: 1000;
    height: 31px;
    width: 31px;
}

#busy-holder
{
    background: transparent;
    width: 100%;
    height: 100%;        
}

Answer 1

Here is a solution using an overlay that inhibits along with material design spinner that you configure one time in your app and you can call it from anywhere.

app.component.html

(put this somewhere at the root level of your html)

<div class="overlay" [style.height.px]="height" [style.width.px]="width" *ngIf="message.plzWait$ | async">
        <mat-spinner class="plzWait"  mode="indeterminate"></mat-spinner>
</div>

app.component.css

.plzWait{
  position: relative;
  left: calc(50% - 50px);
  top:50%;

}
.overlay{
  position: absolute;
  top:0px;
  left:0px;
  width: 100%;
  height: 100%;
  background: black;
  opacity: .5;
  z-index: 999999;
}

app.component.ts

height = 0;
width = 0;
constructor(
    private message: MessagingService
  }
ngOnInit() {
    this.height = document.body.clientHeight;
    this.width = document.body.clientWidth;
  }

messaging.service.ts

import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';

@Injectable({
    providedIn: 'root',
  })
export class MessagingService {
    // Observable string sources
  private plzWaitObservable = new Subject<boolean>();


  // Public Observables you subscribe to
  public plzWait$ = this.plzWaitObservable.asObservable();

  public plzWait = (wait: boolean) => this.plzWaitObservable.next(wait);
}

Some other component

constructor(private message: MessagingService) { }
somefunction() {
    this.message.plzWait(true);
    setTimeout(() => {
            this.message.plzWait(false);
    }, 5000);
}