Hi,
  sorry to bug you again. I let the laptop with external HDMI enabled and internal LCD disabled. After a while external HDMI felt asleep into stand-by mode. But, I could not wake up a screen when touching my keyboard. Not even ctrl+alt+f1 helped (which usually brings me into framebuffer console out off the X11 windows and hence wakes up my screen). Alt+SysRq+S seemed to trigger some disk activity at least. The trick which worked was to close the lid and open again. That at least woke up the internal LCD of the laptop. Although it so far feels like a bug in this case it was better than nothing.

$ dmesg
[  746.362842] [7102]: Lid closed.
[  754.655963] [7102]: Lid opened.
[  765.975115] [7102]: Lid closed.
[ 1256.930836] [7102]: Lid opened.
[ 4909.671002] [7102]: Lid closed.
[ 4914.565392] [7102]: Lid opened.
[ 4940.668204] [7102]: Lid closed.
[ 4948.342004] [7102]: Lid opened.
[ 6309.701087] [7102]: Lid closed.
[ 6317.374851] [7102]: Lid opened.
[ 6475.261162] [7102]: Lid closed.
[ 6483.982278] [7102]: Lid opened.
[12216.573741] sysrq: SysRq : Emergency Sync
[12216.574043] Emergency Sync complete
[12217.030881] sysrq: SysRq : Emergency Sync
[12217.031548] Emergency Sync complete
[12217.639123] sysrq: SysRq : Emergency Sync
[12217.639662] Emergency Sync complete
[12218.115483] sysrq: SysRq : Emergency Sync
[12218.115983] Emergency Sync complete
[12218.462908] sysrq: SysRq : Emergency Sync
[12218.463291] Emergency Sync complete
[12219.533405] sysrq: SysRq : Emergency Sync
[12219.533861] Emergency Sync complete
[12228.142422] [7102]: Lid closed.
[12232.069871] [7102]: Lid opened.
$


  I pressed Fn+F1 and the only available display was the internal LCD. I closed the xfce application. Here is more info. Interestingly, xrandr is still thinking current display size is 1920 x 1200 instead of actual 1366x768.

$ xrandr 
Screen 0: minimum 8 x 8, current 1920 x 1200, maximum 32767 x 32767
LVDS1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 340mm x 190mm
   1366x768      60.05*+  40.01  
   1280x720      59.74  
   1024x768      60.00  
   1024x576      60.00    59.90    59.82  
   960x540       60.00    59.63    59.82  
   800x600       60.32    56.25  
   864x486       60.00    59.92    59.57  
   640x480       59.94  
   720x405       59.51    60.00    58.99  
   680x384       60.00  
   640x360       59.84    59.32    60.00  
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
$

$ xrandr --listmonitors
Monitors: 1
 0: +*LVDS1 1366/340x768/190+0+0  LVDS1
$ xrandr --listactivemonitors
Monitors: 1
 0: +*LVDS1 1366/340x768/190+0+0  LVDS1
$ xrandr --listproviders
Providers: number : 1
Provider 0: id: 0x47 cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 3 outputs: 5 associated providers: 0 name:Intel
$


And because I had 'tail -f .xsession-errors' running here is an update:

UNREF LOADER

(xfsettingsd:7085): xfsettingsd-CRITICAL **: 14:33:52.482: Failed to apply display settings
UNREF LOADER

(xfwm4:7063): xfwm4-WARNING **: 14:34:05.286: output size (1366x768) and logical screen size (1920x1200) do not match

(xfwm4:7063): xfwm4-WARNING **: 14:34:05.300: output size (1366x768) and logical screen size (1920x1200) do not match
UNREF LOADER

(xfsettingsd:7085): xfsettingsd-CRITICAL **: 14:34:06.267: Failed to apply display settings
VALID BACKDROP!
UNREF LOADER


I will attach Xorg.0.log file.