Determines which children of the frame are fixed children in a scrolling frame.
In Figma, fixed children are always on top of scrolling (non-fixed) children. Despite the "Fix position when scrolling" checkbox in the UI, fixed layers are not represented as a boolean property on individual layers. Instead, what we really have are two sections of children inside each frame. These section headers are visible in the layers panel when a frame has at least one fixed child.