My laptop (Lenovo T460p) has an 17-6820HQ CPU with HD530 graphics (plus GeForce
940MX Optimus).  I'm running TW (up-to-date).  The default driver for this is
the modesetting one (via modesetting.ids, 8086191B).

With this driver I am recently seeing increased number of artifacts on fast
screen draw.  They show up as horizontal blocks/lines of different color.  I
did not manage to get a screenshot of this :(

Ways I can trigger them are:
 - change desktop (I use the 'slide' animation).  Quite often then the window 
   title bar of windows sliding in are left with a stripe-box pattern.
 - run an opengl application with optirun and the virtualgl bridge, e.g., 
   optirun -b virtualgl glxspheres.
   The central bouncing ball then is having lots of those bar artifacts.

This is with the compositor disabled (I run plasma5 DE).  They do NOT appear if
the compositor is enabled (but I keep is disabled because kwin_x11 has a very
high probability to crash when toggling it on and off).

They also do not occur when using the intel driver.

A forum user suspected it might be somehow related to Bug #1021803 - but my
understanding of the topic is too low to comment on that :(