Not sure if this bugs belongs to evolution or libgtk-3-0 package, but I
mostly see this with Evolution (randomly also with LibreOffice?), so I
don't think it's a Gtk bug affecting all software.

Consider a system with:

- AMD/APU desktop PC with Samsung 4K display connected via DisplayPort.

- All suspend/sleep modes disabled in systemd sleep.conf.

- Evolution running.

- GNOME desktop session locked (Super-L).

- Monitor powered off, since something (perhaps slight mouse movement?)
tends to disable the power saving mode and activate the backlight of the
monitor every few minutes.

This causes the following log entry to be flooded nearly constantly to the
system log:

  evolution[34996]: gdk_monitor_get_scale_factor: assertion 'GDK_IS_MONITOR (monitor)' failed

The errors come in bursts of dozens of errors, and then there might be a
pause of 1 to 15 minutes, but this pattern does not seem to be
consistent. Anyway, sometimes the log (systemd journal) can grow
gigabytes (wearing the SSD) from this single error if the PC sits idle
for few days and the user forgets to exit from Evolution before locking
his screen.