I'm new to this so I may not be correct. This it the patch that introduced this behaviour for me.
phil@min:~/Development/synaptics.git$ git bisect good
cddab79c408db3b13905a2be72aff4f7bf1406f8 is the first bad commit
commit cddab79c408db3b13905a2be72aff4f7bf1406f8
Date: Thu Mar 1 16:27:01 2012 +1000
Fix scoll increment for non-smooth scrolling (#46617)
Introduced in 2603ad69b997c999404ecc441e0d64ea2cc22018. Previously, the
delta was incremented by (actual delta/scroll dist), button clicks where
then sent while (delta-- >= 1).
After 2603ad69b997c999404ecc441e0d64ea2cc22018, the delta was incremented by
the delta only, scroll dist was used as increment in the smooth scrolling
information. On non-smooth-scrolling servers, the driver would now send
too many events. Restore the old behaviour by dividing by the scroll
distance before entering the button click loop.
:040000 040000 2949f0ddca5e3b158adcf9044eb6c5e3f0bff990 0958387eee6136a5aacdff9db87bbad7cddcdd98 M src