Comment 7 for bug 1790525

Extra info:

1.The syslog at a crash shows next error. Probably memory management at an errormemssage is not correct. Also look at the size 2560 x 1024
Sep 08 00:18:35 Gert2 gnome-shell[2141]: CoglError set over the top of a previous CoglError or uninitialized memory.
                                          This indicates a bug in someone's code. You must ensure an error is NULL before it's set.
                                          The overwriting error message was: Sliced texture size of 2560 x 1024 not possible with max waste set to -1
 Sep 08 00:18:35 Gert2 org.gnome.Shell.desktop[2141]: double free or corruption (fasttop)
 Sep 08 00:18:35 Gert2 org.gnome.Shell.desktop[2141]: GNOME Shell crashed with signal 6

2.It looks like the problem in an older problem. At least at Ubuntu 18.04 with display manager lightdm the same crash occurs if no monitors.xml is present and dual monitor is present.

3.It looks like the problem is caused by the different sizes of the monitors.
At startup gdm3 detects next sizes for laptop-monitor and external monitor:

Sep 9 20:22:54 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (--) intel(0): Output LVDS1 using initial mode 1280x800 on pipe 1
Sep 9 20:22:54 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (--) intel(0): Output VGA1 using initial mode 1280x1024 on pipe 0

If at logon there is no monitors.xml file for the user, gdm3 by default tries to configure the external VGA1-monitor at the right of the laptop LVDS1-monitor. This fails, probably because the vertical size 800 of LVDS1 is smaller than 1024 for VGA1. gdm3 retries, which is causing a loop until VDA1-monitor is disconnected (see next log):

Sep 9 20:23:05 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (II) intel(0): resizing framebuffer to 1280x800
Sep 9 20:23:13 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (II) intel(0): resizing framebuffer to 2560x1024
Sep 9 20:23:13 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (II) intel(0): switch to mode 1280x1024@60.0 on VGA1 using pipe 0, position (1280, 0), rotation normal, reflection none

Sep 9 20:23:18 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (II) intel(0): resizing framebuffer to 1280x800
Sep 9 20:23:27 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (II) intel(0): resizing framebuffer to 2560x1024
Sep 9 20:23:27 Gert2 /usr/lib/gdm3/gdm-x-session[1915]: (II) intel(0): switch to mode 1280x1024@60.0 on VGA1 using pipe 0, position (1280, 0), rotation normal, reflection none

4.If monitors.xml is present with configuration of VGA1 above LVDS1, dual monitors can be used. Also in this case it is not possible to graphically configure via the displays arrangement the configuration with above sizes and VGA1 right of LVDS1.

5. A correct monitors.xml-file does not totally solve the problem.
Frequently the start if Ubuntu hangs completely before showing the logon-screen. It is showing a character screen with next last line.

[ OK ] Started GNOME Display Manager.

Syslog gives next lines then:

Sep 9 00:40:47 Gert2 gnome-shell[1309]: JS WARNING: [resource:///org/gnome/shell/ui/windowManager.js 1573]: reference to undefined property "MetaWindowXwayland"
Sep 9 00:40:47 Gert2 gnome-shell[1309]: Failed to allocate texture: Failed to create texture 2d due to size/format constraints
Sep 9 00:40:47 Gert2 gnome-shell[1309]: CoglError set over the top of a previous CoglError or uninitialized memory.#012This indicates a bug in someone's code. You must ensure an error is NULL before it's set.#012The overwriting error message was: Sliced texture size of 2560 x 1024 not possible with max waste set to -1
Sep 9 00:40:47 Gert2 org.gnome.Shell.desktop[1309]: double free or corruption (fasttop)
Sep 9 00:40:48 Gert2 org.gnome.Shell.desktop[1309]: (EE) failed to read Wayland events: Connection reset by peer
Sep 9 00:40:48 Gert2 gnome-session[1290]: gnome-session-binary[1290]: WARNING: Application 'org.gnome.Shell.desktop' killed by signal 6
Sep 9 00:40:48 Gert2 gnome-session-binary[1290]: WARNING: Application 'org.gnome.Shell.desktop' killed by signal 6
Sep 9 00:40:48 Gert2 gnome-session-binary[1290]: Unrecoverable failure in required component org.gnome.Shell.desktop

6. If at the logon-screen the arrow at upper right corner is clicked (to exit), also the system hangs with character-screen and same kind of error at syslog.