How to hide scrollbars in extensions?

Extensions
2

You probably need to add some positioning or a reset set of rules or something else to get the CSS to achieve what you want.

In this example you added overflow: hidden to the wrong element.

There are a number of solutions to this but it’s just a CSS issue with getting content into a fixed size div. There is no one answer. And theres not enough information to make a suggestion

Personally I tend to apply a reset.css then add a div to body and on that div apply the CSS Rules I need/want for the intended outcome.

Something along the lines of

#id_of_the_div {
    position: absolute;
    top: 0px;
    left: 0px;
    right: 0px;
    bottom: 0px;
    overflow: hidden;
}