@Lucien, my use case is slightly different, so you’ll need to tweak the javascript to your needs by getting the various heights of elements and calculating whether to enable the scroll or not. That being said the general steps will allow you to use a Group Focus and add a scroll bar to it.
Like @Bubbleboy mentioned about other content impacting, you’ll notice in the section above the highlighted text I’m getting the height of my header and subtracting that from the height of the window to ensure my GF does not flow past the bottom of the browser window.
I’m doing something else in mine to set the width of the GF based on the right edge of an element on the screen. See the highlighted text in the screenshot.
Here’s a demo showing the Group Focus where the height is only as large as the browser window, displaying a vertical scroll bar, and the width going to the edge of an element I defined.
