Can I hide/remove the UI from Karma result?

I got another answer.

Putting this in the beforeEach function works well

fixture.debugElement.nativeElement.style.visibility = "hidden";

and it would hide the flashing component when testing.

It looks like your test is adding things directly to the dom, which is non-standard (though there are some instances where this is a good idea). In general, your tests should not attach any dom to the document object (eg- by calling document.querySelector('#foo').appendChild(...). If you create dom elements, you should avoid attaching them to the document.

If you do attach dom elements to the document, the simplest way to remove them is in an afterEach block in your describe spec, like this:

afterEach(() => document.querySelector('#my-element').remove());