fix(autocomplete): auto-highlighted first option not display correctly if the floating label is disabled

[crisbeto]

Fixes the first option not appearing as highlighted if the consumer has set floatLabel="never" and autoActiveFirstOption. It looks like we were depending on the transitioned event from the label to kick off the change detection that shows the highlight.

Fixes #13734.

[jelbourn]

LGTM

[jelbourn]

I see that it's already set up this way, but I just noticed that it's weird for this switchMap to have side effects. No action necessary here, but something to keep in mind.

[thomaspink]

@crisbeto @jelbourn As we only need the detectChanges when firstStable fires (optionChanges does not need it, because it is resulting out of an user action) we could limit it to only the firstStable event. Maybe like so:

return merge(firstStable, optionChanges)
  .pipe(
    // create a new stream of panelClosingActions, replacing any previous streams
    // that were created, and flatten it so our stream only emits closing events...
    switchMap((optionChange) => {
      this._resetActiveItem();
      this.autocomplete._setVisibility();

      if (this.panelOpen) {
        this._overlayRef!.updatePosition();
      }

      // Only detectChanges when firstStable fires
      if (!optionChange) {
        this._changeDetectorRef.detectChanges();
      }

      return this.panelClosingActions;
    }),
    // ...

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[mmalerba]

@crisbeto this somehow broke some tests in google so I had to revert

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