When using hidden titlebar with non-native fullscreen, the window would
lose focus after entering the first command. This occurred because:

1. Shell commands update the window title
2. Title changes trigger reapplyHiddenStyle() 
3. reapplyHiddenStyle() re-adds .titled to the style mask
4. Style mask changes during fullscreen confuse AppKit, causing focus loss

Fixed by adding a guard to skip titlebar restyling while fullscreen is
active, using terminalController.fullscreenStyle.isFullscreen for
proper detection of both native and non-native fullscreen modes.

https://ampcode.com/threads/T-c4ef59cc-1232-4fa5-8f09-c65724ee84d3