Comment 3 for bug 30378

Unfortunately, gnome-screensaver checks the X event queue to determine when an event occurs, and most joystick using programs in ubuntu use the kernel event interface (or legacy /dev/js0) from libsdl.

Regarding the mouse scroll, there is a limitation in X that only one client may trap button clicks at a time, so gnome-screensaver must ignore them in order to allow the program to see them. This is also annoying upstream (see src/gs-watcher-x11.c for the rant).