Comment 1 for bug 108523

Some more investigation pinpointed the problem to the kernel, not the X mouse driver.

In kernel 2.6.17 (which I had in 6.10), dmesg lines related to the mouse were
[17179572.484000] mice: PS/2 mouse device common for all mice
[17179592.040000] logips2pp: Detected unknown logitech mouse model 56
[17179592.568000] input: ImExPS/2 Logitech Explorer Mouse as /class/input/input2

"cat /sys/class/input/mouse0/device/protocol" shows the protocol as "ImExPS/2".

In kernel 2.6.20, I get these instead:
[ 18.320140] mice: PS/2 mouse device common for all mice
[ 18.320606] input: Macintosh mouse button emulation as /class/input/input0
[ 40.291729] input: PS/2 Logitech Wheel Mouse as /class/input/input3

"cat /sys/class/input/mouse0/device/protocol" shows the protocol as "PS/2"
(This despite model 56 supposedly being recognized by the driver since commit e3882bb56e31dbb3950e51c1ba914ef2f445650c)

Unfortunately I won't have time in the near future to bisect this... Hopefully in a few weeks. Meanwhile I will be using kernel 2.6.17.
(Are there instructions anywhere for bisecting kernel bugs in an Ubuntu system? Specifically, how do I run with a custom-built kernel?)