Start a browser, log in to play game. Be under attack.

- just inside the edges of the window, there is red pulsing to warn you of the incoming attack.

Click full screen,

, same.

Click again to return to windowed.

Click F11 for browser full screen - no red pulsing, it can be easy to miss the attack icon above the "Main Chat" tab.

The red pulsing should be visible after having pressed F11.