XFCE4, Intel HD graphics, switching DVI output loses signal

tl;dr:
If you have problems under XFCE4 that X does not come up again after switching your monitor (off and on, or using a KVM switch) then add the following line to the Device section of xorg.conf:
Option "HotPlug" "Off"

This stops xfsettingsd from getting signalled on monitor changes. This will probably work only for single monitor environments, not for setups where you switch from a computer monitor to an entertainment display (TV).

Long version:

I am using two computers and switch back and forth with a KVM switch (using DVI and USB). This used to work fine until recently, when I had to install a new motherboard and hence a new graphics card. The new mobo has an integrated Intel HD4600 graphics chip.

The issue is that I could switch from the Linux PC to the Windows PC without problems, but not back. The monitor would display that no signal was detected and go to power save state. The Linux PC was still running fine. I could blindly log out, and as soon as X was terminated the display came back. So it looked like a graphics issue.

A short search found a bug listing from 2014 concerning NVidia graphics and a possible bug in xfsettingsd of XFCE4:
https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-331/+bug/1308105

The issue seems to be with xfsettingsd. This daemon does get signalled when monitors are switched, but get's lost when coming back to the previously working monitor. Details see thread.

The workarounds suggested in the thread are for NVidia cards and do not work for Intel. What I found is that you can prevent xfsettingsd getting information about changes of the connected monitors by using
    Option "HotPlug" "Off"
in the Device section of xorg.conf.  This helped, I can now switch back and forth as I like.

This will probably not work with multi-monitor arrangements, i.e. when you use a monitor for normal computer work, and another monitor (aka TV) for watching films or so.

Usually current Linux systems do not require xorg.conf files anymore. In case your system does not have such files you can create something like
/etc/X11/xorg.conf.d/20-intel.conf

with the content:

Section "Device"
    Identifier "Intel Graphics"
    Driver "intel"
    Option "HotPlug" "Off"
EndSection

Restart X. YMMV, have fun.