Hi Andre,
   I applied the patch to xfdesktop-4.13.2 on my Gentoo Linux. After a brief testing I have indeed no new core dumps in my /var/dumps/, great. I realized that if I use Fn+F1 to use only my external HDMI display and to turn off the laptop's internal LCD, after lid open my laptop has again enabled LCD screen and using the size of the external HDMI display (1920x1200 instead of its own max 1366x768). I do not recall seeing this before. Could this be caused by your patch?

$ 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.
$

Maybe some checksum could be logged too so one could better discern when the event triggered and to better match it so .xsession-errors file contents? In the below log the messages around "Negative content width -1" were supposedly caused by libreoffice which remembers position o the window from a last execution and placed the window on my laptop indeed somewhere above the left upper corner. I assume that is because it remembered position from my 1920x1200 external HDMI screen while at the moment I was using laptop's internal 1366x768 screen. As there is no "Move window" place around the window luckily the only way to get to the menu of the window is to use "Maximize window" from the Xfce panel.

Anyway, the logged events you added in the patch should bear a timestamp (see below).

$ cat ~/.xsession-errors
/usr/bin/startxfce4: X server already running on display :0
gpg-agent[7061]: WARNING: "--write-env-file" is an obsolete option - it has no effect
gpg-agent[7062]: gpg-agent (GnuPG) 2.2.12 started

(xfce4-session:7009): xfce4-session-WARNING **: 11:12:38.070: gpg-agent returned no PID in the variables

(xfsettingsd:7085): xfsettingsd-WARNING **: 11:12:40.634: Property "/Gdk/WindowScalingFactor" has an unsupported type "guint".

** (xfdesktop:7089): WARNING **: 11:12:41.636: Thumbnailer failed calling GetFlavors

(xfsettingsd:7085): xfsettingsd-CRITICAL **: 11:12:42.185: Stored Xfconf properties disable all outputs, aborting.
Warning:          Unsupported high keycode 372 for name <I372> ignored
                  X11 cannot support keycodes above 255.
                  This warning only shows for the first high keycode.
GNOME_KEYRING_CONTROL=/home/mmokrejs/.cache/keyring-SJ6YUZ
SSH_AUTH_SOCK=/home/mmokrejs/.cache/keyring-SJ6YUZ/ssh
Warning:          Unsupported high keycode 372 for name <I372> ignored
                  X11 cannot support keycodes above 255.
                  This warning only shows for the first high keycode.
Warning:          Unsupported high keycode 372 for name <I372> ignored
                  X11 cannot support keycodes above 255.
                  This warning only shows for the first high keycode.

(wrapper-2.0:7146): Gtk-WARNING **: 11:12:44.220: gtk_widget_size_allocate(): attempt to allocate widget with width -3 and height 26

(wrapper-2.0:7147): Gtk-WARNING **: 11:12:44.222: gtk_widget_size_allocate(): attempt to allocate widget with width -1 and height 28

(wrapper-2.0:7147): Gtk-WARNING **: 11:12:44.222: Negative content width -1 (allocation 1, extents 1x1) while allocating gadget (node border, owner GtkFrame)

(wrapper-2.0:7141): Gtk-WARNING **: 11:12:44.242: Negative content width -1 (allocation 1, extents 1x1) while allocating gadget (node button, owner GtkButton)
VALID BACKDROP!

(wrapper-2.0:7151): Gtk-WARNING **: 11:12:45.183: Negative content width -1 (allocation 1, extents 1x1) while allocating gadget (node button, owner XfceArrowButton)
UNREF LOADER
VALID BACKDROP!
VALID BACKDROP!
UNREF LOADER
UNREF LOADER
VALID BACKDROP!
UNREF LOADER
INVALID BACKDROP!
VALID BACKDROP!
UNREF LOADER
VALID BACKDROP!
UNREF LOADER

(xfsettingsd:7085): xfsettingsd-CRITICAL **: 12:31:29.052: Failed to apply display settings

(xfwm4:7063): xfwm4-WARNING **: 12:31:29.055: output size (1920x1200) and logical screen size (1366x768) do not match

(xfwm4:7063): xfwm4-WARNING **: 12:31:29.056: output size (1920x1200) and logical screen size (1366x768) do not match
UNREF LOADER
UNREF LOADER
UNREF LOADER
VALID BACKDROP!
UNREF LOADER
UNREF LOADER

(xfsettingsd:7085): xfsettingsd-CRITICAL **: 12:31:54.788: Failed to apply display settings
UNREF LOADER
UNREF LOADER

(xfsettingsd:7085): xfsettingsd-CRITICAL **: 12:32:28.566: Failed to apply display settings
UNREF LOADER

(xfwm4:7063): xfwm4-WARNING **: 12:32:58.572: output size (1366x768) and logical screen size (1920x1200) do not match

(xfwm4:7063): xfwm4-WARNING **: 12:32:58.576: output size (1366x768) and logical screen size (1920x1200) do not match
UNREF LOADER
VALID BACKDROP!
UNREF LOADER
$