After a recent upgrade on debian testing (June) I'm seeing white stripes on my screen (see screenshot). Groups of small horizontal white stripes at top of the screen, regularly spaced. They correspond exactly to the positions of the left part of chromium's tabs!! Even if chromium is not on the same desktop.

My laptop is a Dell XPS 15 L521x, with intel core i7.
It also have a nvidia card, with optimus. I'm using bumblebee, and I tried with both nvidia driver and nouveau, and even without bumblebee, the same problem occures.

I also tried to shut down KDE's desktop effects (using Alt-Shift-F12). But the problem remains.

I tried to upgrade the bios, and also intel-microcode, but without effect (the pb may have appeared after upgrading bios or microcode, by the way).

Does someone have an idea about this?
Do you want me to provide more information?

Thank you by advance

The problems seem to be solved if I create the file
/etc/X11/xorg.conf.d/20-intel.conf
with, inside:

Section "Device"
   Identifier  "Intel Graphics"
   Driver      "intel"
   Option      "AccelMethod"  "sna"
   Option      "TearFree" "true"
EndSection


It seems to fix the symptoms I described before and other symptoms that
appeared at the same time (but less frequently):
 * characters disappearing in some windows, for example in emacs, firefox,
chromium tabs and URL bar
 * freezes

​​
​The solution it is change the backend available for accelerating. The
default is uxa as say the manual  $man 4 intel

       Option "AccelMethod" "string"
              Select acceleration method.  There are a couple of backends
available for accelerating the DDX. "UXA" (Unified Acceleration
Architecture) is the mature backend that was intro‐
              duced  to  support  the  GEM  driver  model. It is in the
process of being superseded by "SNA" (Sandybridge's New Acceleration).
Until that process is complete, the ability to
              choose which backend to use remains for backwards
compatibility.  In addition, there are a pair of sub-options to limit the
acceleration for debugging use.  Specify  "off"  to
              disable all acceleration, or "blt" to disable render
acceleration and only use the BLT engine.

              Default: use UXA (render acceleration)


You should create the file "20-intel.conf"  into directory
/etc/X11/xorg.conf.d/ and if this directory not to exist you should create
it and fill with the bellow content

Section "Device"
   Identifier  "Intel Graphics"
   Driver      "intel"
   Option      "AccelMethod"  "sna"
EndSection


problem solved.

​