Function open_window 

pub fn open_window<P: Piece>(
    options: WindowOptions,
    build: impl FnOnce() -> P + 'static,
) -> WindowHandle

Open a secondary window (docs/windows.md): open_window(res::window_options().key("city").size(…), build). options.key names the logical window: when a window with this key is already open it is focused and returned instead of duplicated (the preferences pattern); no key always opens a new one. options.kind says whether it is a regular window or a preferences/About panel. Where the toolkit cannot open windows (Cap::MultiWindow = Unsupported; probe it to adapt chrome) the content presents as a fullscreen cover in the primary window instead, closable the same way.