onEvent() runs on the Hyprland IPC listener thread and mutated the label's
style context (add/remove class) directly, racing the GTK main thread's
drawing and corrupting the heap (double free / corrupted double-linked list).
Follow the Submap pattern: onEvent only stores the layout under the mutex and
emits the dispatcher; update() swaps the CSS class on the main thread, tracking
the previously applied class.
Fixes#4665
Alex
6294ed2520