Angular2 unit testing - why nativeElement has empty CSSStyleDeclaration

I'm trying to test a simple header component that has a button and when being focused - opens a dropdown using just css visibility property.

This is the html:

<nav class="navigation">
    <button type="button">
        <span>click here</span>
    </button>
    <ul>
        <li> Text </li>
        <li> Text </li>
        <li> Text </li>
    </ul>
</nav>

This is the scss style:

button {

    span {
        margin-right: 10px;
    }

    &:focus {

        + ul {
            visibility: visible;
        } 
    }
}

ul {
    position: absolute;
    list-style: none;
    z-index: 1000;
    visibility: hidden;
    transition: visibility 0.1s linear;
    background-color: $color-primary;
}

And this is the test:

it('should open dropdown on focus', () => {

    let button = fixture.debugElement.query(By.css('button'));
    button.triggerEventHandler('focus', null);

    fixture.detectChanges();
    let dropdownElement = fixture.debugElement.query(By.css('ul')).nativeElement;

    console.log(dropdownElement);
    console.log(dropdownElement.style);

    expect(dropdownElement.style['visibility']).toBe('visible');

});

When I run the test I can see that console.log(dropdownElement) exists and that console.log(button.nativeElement) returns the CSSStyleDeclaration object - but ALL the properties have an empty string as a value:

CSSStyleDeclaration {
    alignContent: ""
    alignSelf: ""
    ...
    ...
    ...
    color: ""
    visibility: ""
    ...
    ...
}

So basically what I need is to trigger the focus event on the button and then see if the value of the dropdown's css/style property "visibility" is "visible". I can't figure what's wrong, cause i can see that everything is rendering fine in Karma-Debug but all the style properties are empty... any idea what's the problem?

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