On 04/02/2013 01:32 AM, Chris Howe wrote:
> Hello,
>
> I was running SL6.1 OK but now the display no longer works - On boot
> the top half of the display is readable but the bottom half is
> rectangles of black and the screen.  The mouse won't move into the
> lower half of the screen and produces a trail of black behind it.  If
> I login the menus at the top are accessible but in half the width that
> they should be.
> I know that the monitor is OK as I use a KVM switch for another
> computer - I have also connected the monitor directly to the PC with
> no improvement.

Something very similar happened a while back as a result of the 
"universal access" screen helper/magnifier thingy misbehaving:

http://scientificlinuxforum.org/index.php?showtopic=2048

The solution there was to turn the accessibility/screen-magnifier 
program off (the user hadn't intended to have it running in the first 
place).

You mention a KVM switch... if an accessibility program isn't the 
problem, try wiring your system up without the KVM switch in the middle 
and see if your system can build a workable Xorg config on its own (make 
sure you move/remove xorg.conf first). In the past I've had problems 
where I toggled the KVM switch at a bad time which made SL (and OS X) 
re-probe and get crazy results on multi-screen displays.

Beyond that, this will probably become a graphics driver thread. I would 
check the above two things thoroughly before anything else; usually when 
drivers or Xorg config is wrong X doesn't start at all, so it makes me 
think this is more a screen effect gone wrong or a probe/detection issue 
with X.