Bug 86794

Almost once a day X11 hangs. An other issue is that the windows are moving more ore randomly up and down (from foreground to background). Very often is at the window position only a shadow of the window frame filled with the pattern of the desktop background. 

If the desktop is hanging it is sometimes possible to switch to console with ALT-CTL-F1, sometimes I have to login via ssh to check the log files. Often I 'dmesg' reports thne something like:
...
[175947.652266] nouveau E[   PFIFO][0000:01:00.0] read fault at 0x00014a2000 [PAGE_NOT_PRESENT] from PGRAPH/GPC0/TEX on channel 0x003fb74000 [gnome-shell[27277]]
[175947.652281] nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 5, recovering...
[175947.652315] nouveau E[  PGRAPH][0000:01:00.0] TRAP ch 5 [0x003fb74000 gnome-shell[27277]]
[175947.652331] nouveau E[  PGRAPH][0000:01:00.0] GPC0/TPC0/TEX: 0x80000049
[175947.652340] nouveau E[  PGRAPH][0000:01:00.0] GPC0/TPC1/TEX: 0x80000049
[176136.543763] nouveau E[Xorg[26420]] failed to idle channel 0xcccc0000 [Xorg[26420]]
[176151.552203] nouveau E[Xorg[26420]] failed to idle channel 0xcccc0000 [Xorg[26420]]
[176151.552337] nouveau E[   PFIFO][0000:01:00.0] read fault at 0x000001b000 [PAGE_NOT_PRESENT] from PFIFO/BAR_READ on channel 0x003fc60000 [unknown]
[176153.985794] nouveau E[     DRM] GPU lockup - switching to software fbcon


'journalctl -xb | grep -A 6 "NVIDIA\|NOUVEAU\|nouveau E"' says:
...
Nov 01 08:32:51 impala gdm-Xorg-:0[26420]: (II) NOUVEAU(0): Monitor name: W3000H
Nov 01 08:32:51 impala gdm-Xorg-:0[26420]: (II) NOUVEAU(0): Monitor name:
Nov 01 08:32:51 impala gdm-Xorg-:0[26420]: (II) NOUVEAU(0): EDID (in hex):
Nov 01 08:32:51 impala gdm-Xorg-:0[26420]: (II) NOUVEAU(0):         00ffffffffffff001e6dea754d810600
...
Nov 02 23:54:39 impala kernel: nouveau E[   PFIFO][0000:01:00.0] read fault at 0x00014a2000 [PAGE_NOT_PRESENT] from PGRAPH/GPC0/TEX on channel 0x003fb74000 [gnome-shell[27277]]
Nov 02 23:54:39 impala kernel: nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 5, recovering...
Nov 02 23:54:39 impala kernel: nouveau E[  PGRAPH][0000:01:00.0] TRAP ch 5 [0x003fb74000 gnome-shell[27277]]
Nov 02 23:54:39 impala kernel: nouveau E[  PGRAPH][0000:01:00.0] GPC0/TPC0/TEX: 0x80000049
Nov 02 23:54:39 impala kernel: nouveau E[  PGRAPH][0000:01:00.0] GPC0/TPC1/TEX: 0x80000049


If the the window is randomly shifted randomly up and down 'journal -xb' reports
...
Nov 26 08:16:45 impala gnome-session[31470]: Fensterverwalter-Warnung:Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x320000d (sshd_confi)
Nov 26 08:16:45 impala gnome-session[31470]: Fensterverwalter-Warnung:meta_window_activate called by a pager with a 0 timestamp; the pager needs to be fixed.
...
Nov 26 08:22:57 impala gnome-session[31470]: Fensterverwalter-Warnung:last_focus_time (49506834) is greater than comparison timestamp (49506833).  This most likely represents a buggy client sending inaccurate timestamps 
...



The errors are also file as gentoo bugs at https://bugs.gentoo.org/show_bug.cgi?id=530780 and https://bugs.gentoo.org/show_bug.cgi?id=528100.