You have two different events, one is mousedown and another is click.
The e.stopPropagation() only works if both of the events are of the same kind.
You can change the input like this to work as expected:
<input #inputBox matInput (click)="fireEvent($event)" max-width="12" />
Live example:
https://stackblitz.com/edit/angular-material-basic-stack-55598740?file=app/input-overview-example.ts