To create a fixed element, select a layer or Hotspot and check the Fix position when scrolling option in the Prototyping section of the Inspector. Alternatively, choose Prototype > Fix Layer Position when Scrolling in the menu.
This is handy when you want to fix a row of tabs at the bottom of an app design, or have a floating button on a website, for example.
How to create scrolling Artboards
If you want to create a prototype with scrolling elements, you’ll need to choose an Artboard Template and increase its height. If you don’t use a template and draw a custom Artboard instead, your prototype won’t scroll and will appear zoomed out in the app, on the web app and in the Sketch iPhone app.
To create a scrolling prototype from an Artboard Template, select your template and increase its height on the canvas. When you do this the template name in the Inspector changes to include the word Resized. Sketch comes with Artboard Templates for common devices but you can also create your own Artboard Templates.