Function open_window 

pub fn open_window(
    key: Option<&str>,
    options: WindowOptions,
    kind: WindowKind,
    build: impl FnOnce() -> AnyPiece + 'static,
) -> WindowHandle

Open a secondary window (docs/windows.md). 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); None always opens a new one. 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.