( I'm upstream of Castle Game Engine and Castle Model Viewer. ) This should be solved once we release new Castle Game Engine, and it will be packaged in Debian along with new Castle Model Viewer.
The latest Castle Game Engine has `TCastleWindow` backend using GTK 3 by default (not GTK 2). `TCastleWindow` is a class used to initialize window in Castle Model Viewer and most CGE examples. So, simply recompiling the application using the latest Castle Game Engine will fix this issue for Castle Model Viewer. See https://castle-engine.io/wp/2026/02/09/upgrade-to-gtk-3/ . We plan soon (February 2026) to do new releases of Castle Game Engine and Castle Model Viewer. Regards, Michalis
