Fixed elements are layers or Hotspots that will stay in one place while the rest of your content moves behind them, to mimic a scroll view in your prototypes. This lets you add static header and footer elements to your designs, and preview them as they will appear in the final product.
To create a fixed element, just select the layer or Hotspot you want to keep in one place and select the Fix position when scrolling checkbox in the Prototyping section of the Inspector or go to Prototyping › Fix Layer Position when Scrolling in the menu.
When a layer is fixed, it will appear above every other layer when viewing a prototype, no matter it’s layer order. Be sure to also fix any Hotspot layers if they aren’t already grouped with the content you want to remain fixed.
To make a fixed element scroll again, simply deselect the Fix Position when Scrolling checkbox in the Inspector.
When fixing the position of elements that you want to appear at the bottom of your viewport — such as Android’s floating action button, or an iOS tab bar — you don’t need to do anything special to your design. If you are using a resized Artboard preset, you can position these elements at the bottom of the Artboard, below the device’s expected viewport, and they will appear in the exact position you would expect when previewing your prototype.