Zotero is based on Firefox, and, like in Firefox, smooth scrolling is enabled by default. You can see this in the tag selector in the bottom left, which will scroll smoothly if your system is set up to do that. However, "trees", such as the history/bookmarks list in Firefox or the collections/items lists in Zotero, still get scrolled a line at a time, and as far as I know that can't be changed. (I'm not really sure why you would ever want to see only part of a list row, and I guess Mozilla feels the same way.)