Video header / youtube / resizing

Hi,

if I use a video header in this 2017 theme AND the video source is youtube, I have a problem with the behaviour of the video iframe when resizing the browser window (or when loading the page in a browser window which is not fullcreen on a standard 16:9 monitor). There are – depending on the aspect ratio of the browser window – black bars above and below the video or on the left and the right side. It doesn’t work like object-fit: cover (like it does when using a local video my own server), but rather like object-fit: contain.
Is there any solution to this? I tried different changes in the css and different solutions proposed on the web, but nothing worked.

Has anyone found a solution? Any help is greatly appreciated!