We have an application that is using Theme Cosmos, and we heavily use the preview panel. We're already using a layout group (accordeon) to condense information, but in some cases a scrollbar is desirable. For example:

One solution would be to override the .slide-in-summary class (or a more specific selector, i.e. div[node_name="pxMainPageSideInContainer"].slide-in-summary) with CSS, and set overflow to 'scroll' instead of 'auto'. But, I'm curious if future release provide an extension for this or if there are better solutions than overriding the CSS in order to avoid breaking upgrade changes.