In Nautilus, we have a compact icon view that scrolls horizontally but not vertically.

Many people have mouses that only have vertical scroll wheels, but they want to use the vertical scroll wheel to scroll horizontally (bug 537552).

On the other hand, there are 3D scrolling devices that have both vertical and horizontal scroll wheels / axes, but for these devices it is not a good idea to treat a vertical scroll event as a horizontal scroll.

It would be great if GTK+ detected

A) the scroll event source device and its number of scrollable axis
B) the target widget scrollbar visibility

C1) for devices with both axes, or a widget with both scroll bars:
  horizontal device event -> horizontal GTK+ event
  vertical device event -> vertical GTK+ event
C2) for devices with one axis and a widget with one scroll bar:
  any device event -> GTK+ event for visible scroll bar

The desired behavior could be implemented in GtkScrolledWindow, which would cover 98% of the scroll bar use-cases.

One last thing: We have GdkDevice, and GdkAxisUse structures, but while
  GDK_AXIS_X
  GDK_AXIS_Y
  GDK_AXIS_WHEEL
are available, I can't find 3D devices, or wheel entries i.e.
  GDK_AXIS_Z
  GDK_AXIS_WHEEL_X
  GDK_AXIS_WHEEL_Y
  GDK_AXIS_WHEEL_Z

Maybe we don't even support devices with muliple scroll wheels?