Function make_on_toggle_expand 

fn make_on_toggle_expand(
    expanded: RwSignal<bool>,
    displays: RwSignal<Vec<DisplaySourceView>>,
    windows: RwSignal<Vec<WindowSourceView>>,
    error_message: RwSignal<Option<String>>,
    active_source_id: RwSignal<Option<String>>,
) -> impl Fn(MouseEvent) + Copy + Send + Sync + 'static

Closure factory for the expand-dropdown button.