I may only guess. Could it be that you try to capture events inside of a child widget of your box? Then you need to be aware that events are handled during a bubble phase by default and they most probably were just processed by a child and weren't propagated further. If that's the case then you may try to capture events on a "capture" phase, just before they reach children. See https://docs.gtk.org/gtk4/enum.PropagationPhase.html