Bug#917969: lightdm: crashes X on dm-tool switch-to-greeter with multiple displays

[Rob Browning <rlb@defaultvalue.org>]

Package: lightdm
Version: 1.26.0-3

If I have more than one display active[1], running switch-to-greeter
causes the current X session on vt7 to crash and lightdm never comes
back up on that (original) terminal, at least not until I restart it.
The new lightdm login does appear on vt8.

While I'm not entirely sure this is an issue with lightdm, I've started
trying to narrow it down, and if I just launch X sessions for two
different users manually on say vt3 and vt4 via startx, terminal
switching between them just fine.

This problem is easy to reproduce, so I'm happy to help with the
diagnosis if I can.

I've observed this across any number of kernel and other package
versions on another latptop, and on this laptop I currently have:

  linux-image: 4.19.12-1
  lspci: 00:02.0 VGA compatible controller: Intel Corporation UHD Graphics 620 (rev 07)

[1] So far I've observed the crashes with intel graphics on two
    different laptops, and I just did a bit more careful testing that
    indicates switching works fine when just a single external
    thunderbolt/usb3->display-port connection is active, but crashes if
    either an additional hdmi external connection is active, or if the
    laptop's internal display is active.  I *think* it also works fine
    if only the internal display is active, but I'll have to
    double-check that later.

Thanks
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org
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