This bug is caused by xfce4. I removed the config files under ~/.config and the desktop images appeared and the dimming went away. The dimming was probably caused by a lack of a desktop image on the second monitor.
The missing desktop images was caused by a change in the /home/<user>/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml. Before the upgrade, xfce4 was using the monitorDVI-0 and monitorDVI-1 sections. After the xserver-xorg package upgraded, xfce4 started using the monitor0 and monitor1 sections. These sections did not use my desktop background images. For some reason, xfce4 is working differently after the xserver-xorg upgrade. The xrandr shows two monitors DVI-0/DVI-1 before and after the upgraded. I do not understand why xfce4 switched from monitorDVI-0|1 to monitor0|1. Perhaps this bug should be moved to xfce4? I am using the latest xfce4 at 4.12.2.
