After upgrading to kernel 4.13.0-26-generic "3/4" part of screen is unreadable. It is both in xorg/xfce and console (ctrl+alt+F1). Everything works perfect when I reboot my laptop with older 4.10.0-42-generic kernel. I'm not sure whether it is Xorg bug or kernel graphics driver bug. The unvisible 3/4 part of the screen seems to be fully operational. I may open an application window in it and drag it to the visible 1/4 part of the screen. Besides the screen display issue the system seems to run properly. I was able to report this bug while running the 4.13.0-26-generic kernel. The laptop is Samsung N130.
ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: xorg 1:7.7+13ubuntu3
ProcVersionSignature: Ubuntu 4.13.0-26.29~16.04.2-generic 4.13.13
Uname: Linux 4.13.0-26-generic i686
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: i386
CurrentDesktop: XFCE
Date: Thu Jan 11 18:29:04 2018
InstallationDate: Installed on 2017-12-18 (24 days ago)
InstallationMedia: Xubuntu 16.04.3 LTS "Xenial Xerus" - Release i386 (20170801)
SourcePackage: xorg
Symptom: display
UpgradeStatus: No upgrade log present (probably fresh install)
This does not happen in Xubuntu 18.04 any more
Linux 4.15.0-72-generic
I treat it as fixed