Comment 10 for bug 445100

ok, searching launchpad and googling a lot,
I found this bug is an effect caused by bug: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/438000
also reported into bug on freedesktop.org: http://bugs.freedesktop.org/show_bug.cgi?id=22076

As you can see the real problem is a bug/limitation on mesa-lib, you can avoid it by putting your external Monitor (VGA1) "below" your current screen (LDVS1),

NOTE: You've always "2048x2048 maximum 3D texture size" limitation so check if your vertical size will not overcome 2048, example: In my case vertical size is: 640 + 1024 = 1664 that doesn't overcome 2048 :-)

I've tested it and now it works great with compiz too,

this an example script that you can use to switch correctly on (without freeze) your external monitor:

#!/bin/bash

lvdsres="1024x600"
vgares="1280x1024"

if [ -e ~/.vgaconnected ]
then
xrandr --output VGA1 --off --output LVDS1 --mode $lvdsres && rm ~/.vgaconnected
else
xrandr --output LVDS1 --mode $lvdsres --output VGA1 --mode $vgares --rate 60 --below LVDS1 &> ~/.xrand.log && touch ~/.vgaconnected
fi

Let me know if this solution solved your problems too!

bye.