Function create_owned_popup 

fn create_owned_popup(
    owner_hwnd_raw: *mut c_void,
    window_label: &str,
    x: i32,
    y: i32,
    width: i32,
    height: i32,
) -> Result<*mut c_void, String>

CreateWindowExW wrapper that produces the owned WS_POPUP + WS_EX_TOOLWINDOW HWND used as the floating-pane outer shell.

The class is registered once per process; subsequent calls reuse the registered atom.