message: Return `Mode::Hidden` in `window::Action::FetchMode`
  ... when the window is not visible.
diff: @@ -635,7 +635,11 @@ pub fn run_command<A, E>(
  ));
  }
  window::Action::FetchMode(tag) => {
  - let mode = conversion::mode(window.fullscreen());
  + let mode = if window.is_visible().unwrap_or(true) {
  + conversion::mode(window.fullscreen())
  + } else {
  + window::Mode::Hidden
  + };
  proxy
  .send_event(tag(mode))
  