Confine the cursor to a window

Set the CursorOptions on the Window to CursorGrabMode::Confied.

use bevy::prelude::*;
use bevy::window::{CursorGrabMode, CursorOptions};

fn main() {
  let window = Window {
    cursor_options: CursorOptions {
      grab_mode: CursorGrabMode::Confined, // Enable cursor confinement
      ..default()
    },
    ..default()
  };

  App::new()
    .add_plugins(DefaultPlugins.set(WindowPlugin {
      primary_window: Some(window),
      ..default()
    }))
    .run();
}

This currently has poor cross-platform support and won't work on macOS.