gtk_window_set_decorated alone doesn't work on KDE Wayland — KWin adds server-side decorations regardless. Now also calling gdk_window_set_decorations(gdk_win, 0) after realize and before first frame show, which communicates directly with the compositor via GDK's Wayland layer. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>