feat(autocomplete): Automatically close (resp. open) the panel when the triggering 'input' is no longer visible (becomes visible again)

[tinesoft]

Feature Description

Currently, when the input element that triggered the opening of the dropdown panel is no longer visible in the viewport (after a scroll up or down), the panel remains still visible, which is weird, as it appears "floating around in the air" and no longer "attached" to its peer input.

Before scrolling up

After scrolling up (the triggering 'Assignee' input is now off the viewport, higher up)

Using the Intersection Observer API, we could determine is the input is still visible inside the Viewport, and close the panel if it is not.

Additionally, when the input becomes visible again, if the input element still has focus, we could open the panel again.

Use Case

When using the Matautocomplete panel inside a scrollable container and that the input can become visible/hidden after scrolling the viewport

[crisbeto]

It's not exactly what you're asking for, but we support something similar:

import {MAT_AUTOCOMPLETE_SCROLL_STRATEGY} from '@angular/material/autocomplete';
import {Overlay} from '@angular/cdk/overlay';

@NgModule({
  providers: [{
    provide: MAT_AUTOCOMPLETE_SCROLL_STRATEGY,
    deps: [Overlay],
    useFactory: (overlay: Overlay) => {
      return overlay.scrollStrategies.close({threshold: 50});
    },
  }]
})
class YourModule {}

This will close the overlay if the user scrolls away more than 50px.

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