For a UI element that must remain fixed at the top of the screen while the rest of the content scrolls underneath it, what specific Figma feature is applied to achieve this behavior?

The specific Figma feature applied to make a UI element remain fixed at the top of the screen while the rest of the content scrolls underneath it is called 'Fix position when scrolling'. When you select a layer or group in Figma and go to the Prototype tab in the right-hand inspector panel, you'll find this setting. By default, layers and elements scroll with the rest of the content on the canvas. However, when you check the 'Fix position when scrolling' box for a particular element, you are instructing Figma to treat that element differently during the prototyping or presentation of your design. This means that no matter how far you scroll down the rest of the page content in the preview, the element with this setting enabled will always stay in its original position at the top of the viewport, which is the visible area of the screen. This is crucial for elements like navigation bars or headers that need to be consistently accessible to the user. For instance, imagine a website header containing a logo and navigation links. You would select this header group in Figma, navigate to the Prototype tab, and enable 'Fix position when scrolling'. When you then present your design prototype, the header will remain visible at the top as the user scrolls through the main body content of the page.