When the widnow changes state, like it goes fullscreen, maximized, etc, winit should broadcast window state. I'd suggest to bundle it with resize events, and probably rename it.

pub struct WindowAttrs {
  maximized: bool,
  fullscreen: bool,
  minimized: bool,
}

enum WindowEvent {
  Configure(size: PhysicalSize<u32>, attrs: WindowAttrs)
}

This api is essential to know when the window gets fullscreened and disable things like opacity or change buttons if they're drawn by the window. Also the app could change layout in fullscreen (hide status bar, like browser).