Some man pages.

man/man3/mousescrollsize.3

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+.TH MOUSESCROLLSIZE 3
+.SH NAME
+mousescrollsize \- compute mouse scroll increment
+.SH SYNOPSIS
+.B #include <draw.h>
+.PP
+int	mousescrollsize(int maxlines)
+.SH DESCRIPTION
+.I Mousescrollsize
+computes the number of lines of text that should be scrolled
+in response to a mouse scroll wheel click.
+.I Maxlines
+is the number of lines visible in the text window.
+.PP
+The default scroll increment is one line.
+This default can be overridden by setting the
+.B $mousescrollsize
+environment variable to an integer, which specifies
+a constant number of lines, or to a real number
+followed by a percent character, indicating that the
+scroll increment should be a percentage of the total
+number of lines in the window.
+For example, setting 
+.B $mousescrollsize
+to 
+.B 50%
+causes a half-window scroll increment.
+.PP
+.I Mousescrollsize
+is used by
+.IR 9term (1)
+and
+.IR acme (1)
+to set their scrolling behavior.
+.SH SOURCE
+.B /usr/local/plan9/src/libdraw/scroll.c
+.SH SEE ALSO
+.IR 9term (1),
+.IR acme (1)
+.SH BUGS
+.I Libdraw
+expects up and down scroll wheel events to be expressed as clicks of mouse buttons 4 and 5,
+but the XFree86 default is to ignore the scroll wheel.
+To enable the scroll wheel, change your
+.B InputDevice
+section of
+.B XF86Config-4
+to look like:
+.IP
+.EX
+Section "InputDevice"
+	Identifier	"Mouse0"
+	Driver	"mouse"
+	Option	"Device" "/dev/psaux"
+
+	# next four lines enable scroll wheel as buttons 4 and 5
+	Option	"Buttons" "5"
+	Option	"Emulate3Buttons" "off"
+	Option	"Protocol" "ImPS/2"
+	Option	"ZAxisMapping" "4 5"
+EndSection
+.EE