Function XPUFixedLayout 

pub unsafe extern "C" fn XPUFixedLayout(
    inMessage: XPWidgetMessage,
    inWidget: XPWidgetID,
    inParam1: isize,
    inParam2: isize,
) -> c_int

XPUFixedLayout

This function causes the widget to maintain its children in fixed position relative to itself as it is resized. Use this on the top level ‘window’ widget for your window.