Support is provided for a fourth button for those pointing devices which have four buttons. Pressing button 4 does nothing except update the coordinates displayed on-screen. No action is performed. This can be simulated by holding the Ctrl, Shift, and Alt keys while pressing button 1.
Under Unix/Linux, the GTK+ user interface provides support for mouse wheels, by assuming that up and down clicks are mapped to button 4 and 5 presses. In order to use the mouse wheel, the X server must enable this function. For FreeBSD and Linux, this is enabled in the XF86Config file in /usr/X11R6/lib/X11, by adding the ``ZAxisMapping'' keyword to the ``Pointer'' section, as shown in the example below.
Section "Pointer" Protocol "Auto" Device "/dev/psm0" ZAxisMapping 4 5 EndSection
Any window that has scroll bars can be scrolled by moving the pointer over a scroll bar and turning the mouse wheel. The text windows and help viewer windows in Xic respond to the mouse wheel by scrolling when the pointer is in the text window, as well as over the scroll bar.