Function check_window_sidebar_button 

pub fn check_window_sidebar_button<A: ResourceApp>(
    open: impl FnMut() -> Result<HeadlessApp<A>, LaunchError>,
) -> Result<(), String>

Open the app with open in each of SIDEBAR_BUTTON_WINDOWS, shown in the desktop’s window bar, and click the bar’s sidebar button three times there: every click flips the sidebar, on the bar, in the window’s shell and on screen (FrameHarness::check_window_sidebar_button).