The problems seem to be solved if I create the file
/etc/X11/xorg.conf.d/20-intel.conf
with, inside:
Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "AccelMethod" "sna"
Option "TearFree" "true"
EndSection
It seems to fix the symptoms I described before and other symptoms that appeared at the same time (but less frequently):
* characters disappearing in some windows, for example in emacs, firefox, chromium tabs and URL bar