OK - here's a demo which shows the issue: Compile and run, just drag the scrollbar into the center and then quickly drag it up and down by 1 cm. You'll see occasional flickering of the first and last visible lines. Confirmed with Laz trunk/fpc 2.6.4 and with Laz 1.4/fpc 2.6.4. I can also confirm your observation that this did not occur earlier - I only have Laz 1.2 now, but there is no flicker with this version.
I looked through the svn logs, but did no find an obvious change.
ncs-sniper, please file a bug report, you can attach this demo if you want.