Multi-monitor: Fullscreen applications spanning over both monitors [not intended]

This problem started to occur after a clean install of Ubuntu 12.04 final, it wasn't present in the beta2, Ubuntu 11.10 and 10.10.

I have a multimonitor setup (two monitors, different resolutions) with a stock open-source Radeon driver on AMD 5670. The problem I am having is, when the fullscreen mode is selected (eg. in a game, glxgears -fullscreen, but not Firefox, LibreOffice, VLC), the applications always spans across both monitors instead of just the current screen. So the effective fullscreen desktop size is seen as one monitor with 1680+1280x1050+1024 (2960 x 1050) instead of two monitors with 1920x1050 + 1280x1024. For example, running "glxgears -fulscreen" command from console on main monitor should maximize the test on main monitor only (1680x1050), but instead it spans the test over both monitors (not mirroring it) with fullscreen resolution of 2960x1050). This is unintended and not a case in previous versions of Ubuntu.

I have no idea how to appprehend this problem, there is no xorg.conf in "/etc/X11/" folder by default.

glxinfo | grep -i "opengl\|direct\|server\|client" && lspci | grep -i vga && uname -a
direct rendering: Yes
server glx vendor string: SGI
server glx version string: 1.4
server glx extensions:
client glx vendor string: Mesa Project and SGI
client glx version string: 1.4
client glx extensions:
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD REDWOOD
OpenGL version string: 2.1 Mesa 8.0.2
OpenGL shading language version string: 1.20
OpenGL extensions:
01:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Redwood [Radeon HD 5670]
Linux CENTURION 3.2.0-24-generic #37-Ubuntu SMP Wed Apr 25 08:43:22 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

xrandr --current
Screen 0: minimum 320 x 200, current 2960 x 1050, maximum 8192 x 8192
HDMI-0 disconnected (normal left inverted right x axis y axis)
DVI-0 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 470mm x 300mm
   1680x1050 60.0*+
   1600x1000 60.0
   1280x1024 75.0 60.0
   1440x900 59.9
   1280x960 60.0
   1152x864 75.0
   1024x768 75.1 60.0
   832x624 74.6
   800x600 75.0 60.3
   640x480 75.0 60.0
   720x400 70.1
VGA-0 connected 1280x1024+1680+26 (normal left inverted right x axis y axis) 376mm x 301mm
   1280x1024 60.0*+ 75.0
   1280x960 60.0
   1152x864 75.0
   1024x768 75.1 70.1 60.0
   832x624 74.6
   800x600 72.2 75.0 60.3 56.2
   640x480 72.8 75.0 66.7 60.0
   720x400 70.1