Desktop effects stopped working on Ubuntu 9.04

[Ori Berger]

Ori Idan wrote:
> I have an Ubuntu 9.04 laptop with Intel 945 graphics adapter.
> Desktop effects worked until this morning when I connected it to an 
> external monitor.
> After disconnecting the external monitor and restarting the laptop, 
> desktop effects stopped working.
> Does someone have an idea how to reenable them?
It's a known bug in the Intel driver: You placed the external screen to 
the left/right of the laptop screen, causing the effective screen width 
to be one that the Intel Driver does not support 3D acceleration for, 
thereby causing compiz to turn off effects.

If effects won't turn on, check /etc/X11.conf under section "Screen" 
subsection "Display" to see it doesn't still list a too-wide screen 
(your laptop is probably 1024 or 1280 pixels wide). When this happened 
to me, I deleted the "Display" altogether and let Ubuntu re-figure it 
out itself, but a "sudo dpkg-reconfigure xserver-xorg" might be helpful.

And from now on, when you connect an external monitor, just be sure to 
place the screen above/below the laptop screen rather than to the 
left/right of it.