title: Cannot re-enable material autocomplete panel after disabling it
body: <p>I think I have found a bug in <code>mat-autocomplete</code> (or not understanding how it works) but I want to run it by you all first.</p>
  
  <p>In the TS I tap the the stream of data for the filtered list, if the list is more than 6 I do not want the autocomplete panel to show. </p>
  
  <p>There is an <code>@Input</code> called <code>AutocompleteDisabled</code> that takes a <code>boolean</code> according to the <a href="https://material.angular.io/components/autocomplete/api" rel="nofollow noreferrer">docs</a>:</p>
  
  <blockquote>
    <p>Whether the autocomplete is disabled. When disabled, the element will
    act as a regular input and the user won't be able to open the panel.</p>
  </blockquote>
  
  <p>I have tried subscribing from the HTML which didnt change the result. I even tried making a checkbox that flipped the value from true to false but once I set the checkbox to false, even if I make it true again the autocomplete panel fails to work. </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>@Component({
    selector: 'app-sample',
    templateUrl: './app-sample.component.html',
    styleUrls: ['./app-sample.component.styl']
  })
  export class AppSampleComponent implements OnInit {
    selectedOption = new FormControl('');
    disablePanel = false;
    filteredOptions: Option[] = [];
    private options: Option[] = [];
  
  
    constructor(private myService: MyService) { }
  
    ngOnInit() {
      this.myService.getOptions().subscribe(data =&gt; this.options = data.carriers);
      this.selectedOption.valueChanges.pipe(
        startWith&lt;string | Option&gt;(''),
        map(value =&gt; typeof value === 'string' ? value : value.name),
        map(name =&gt; name ? this.filterByName(name) : this.options.slice()),
        tap(filtered =&gt; {
          // If ever this evelauates to true, the panel stops appearining and when it evaluates back to false
          // it is as if all of auto-complete stops working.
          this.disablePanel = filtered.length &gt; 6;
        }
      ).subscribe(filtered =&gt; this.filteredOptions = filtered);
    }
  
    private filterByName(value: string): Option[] {
      return this.options.filter( =&gt; option.name.toLowerCase().indexOf(value.toLowerCase()) === 0);
    }
  
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;mat-form-field&gt;
    &lt;mat-icon matSuffix&gt;search&lt;/mat-icon&gt;
    &lt;input [formControl]="selectedOption"
           placeholder="Select and Option"
           matInput
           [matAutocompleteDisabled]="disablePanel"
           [matAutocomplete]="auto"&gt;
    &lt;mat-autocomplete #auto="matAutocomplete" [displayWith]="displayFn"&gt;
      &lt;mat-option *ngFor="let option of filteredOptions " [value]="option"&gt;
        {{option.name}}
      &lt;/mat-option&gt;
    &lt;/mat-autocomplete&gt;
  &lt;/mat-form-field&gt;</code></pre>
  </div>
  </div>
  </p>