Make mat-autocomplete scroll behavior match mat-select.

[daicoden]

Please describe the feature you would like to request.

Disable the ability to scroll the page when the autocomplete options are displayed, so autocomplete works the same as mat-select.

What is the use-case or motivation for this proposal?

Currently when you use mat-autocomplete, the options do not stick to the dropdown field - reported in #12500. The mat-select behavior works great, so could we copy it for mat-autocomplete?

Is there anything else we should know?

Looking through the mat-select code, there's 1000 lines of code related to scrolling. I could work on a PR with some pointers to where the behavior is located.

[andrewseguin]

Currently the dropdown does scroll with the autocomplete, but it is still valid to request a change in scrolling behavior if you want it locked like the select.

[tommyc38]

Mine isn't scrolling and looks terrible when it hangs while the rest of the page scrolls. Any ideas on what might cause this???

[DavideDelbianco]

The autocomplete panel scrolls only by listening to body and html scoll events, if you have it inside another scrollable parent element add to that element the directive cdkScrollable and import { ScrollingModule } from "@angular/cdk/scrolling";
This will add that element to those watched for scroll events.

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