So fluid mode with bubble responsiveness just conflict with each other so I took it out a while back because of that. You can try it though if you want to, although I don’t think it will make much difference. To give that a try, you can run the execute method action and enter:
fluid(true);
Failing that, make sure the player and any groups and your page are not set to fixed width then set the aspect ratio setting to 4:3 and the object-fit setting to cover.
If none of that works how you need it to, I’ll play with the CSS. I have had this type of question come up a few times in the past and I’ve put controls in to make it work, I just need to double check what I’d done which I’ll do if you still have troubles.