When I attach the HDMI cable the laptop screen turns off, leaving only the external monitor on according to my configuration. When I detach the HDMI cable the laptop screen doesn't turn on. This happens both when I only attach the HDMI external monitor and also when I attach it while the laptop is connected to a docking station with another external monitor. Another strange thing is that sometimes, when I connect the HDMI external monitor and I open the configuration dialog, the HDMI output is marked as disabled even though is in fact enabled and working. Kscreen seems to not produce kscreen.log, so I cannot provide that type of extra information. Kscreen 4.9.4 Kernel 4.9.22 Graphics: Card: Intel Haswell-ULT Integrated Graphics Controller Display Server: X.Org 1.19.3 driver: intel Resolution: 1920x1080@60.02hz, 1920x1080@60.00hz GLX Renderer: Mesa DRI Intel Haswell Mobile GLX Version: 3.0 Mesa 17.0.3
This also happened to me few times but when I tried to reproduce it, I was not successful. Due to my main problem in bug 381515 I do have kscreen.log so when it occurs again I will attach my kscreen.log here.
I found the cause - it was caused by my change :O of /etc/X11/xorg.conf.d/20-intel.conf. The problem was Option "AccelMethod" "uxa" and when I changed it back to Option "AccelMethod" "sna" it started to work again. The reason I changed this configuration was an attempt to play with with hw acceleration in Firefox which btw did not help. https://forum.manjaro.org/t/external-monitor-does-not-wake-up/41985/37?u=peke
I confirm it has been solved for me as well. I indeed did the same things reported by Peter, so the cause could have been the same.